[8.16](backport #42144) Revert the Ubuntu version update

[mergify]

Due to ECE compatibility issues we have to switch back to 20.04.

How to test this PR locally

PLATFORMS=linux/arm64 PACKAGES=docker mage package

Before the change

docker.elastic.co/beats/filebeat 8.17.1 19e23b0fa142 356MB

docker history

IMAGE          CREATED              CREATED BY                                      SIZE      COMMENT
19e23b0fa142   About a minute ago   CMD ["-environment" "container"]                0B        buildkit.dockerfile.v0
<missing>      About a minute ago   ENTRYPOINT ["/usr/bin/tini" "--" "/usr/local…   0B        buildkit.dockerfile.v0
<missing>      About a minute ago   WORKDIR /usr/share/filebeat                     0B        buildkit.dockerfile.v0
<missing>      About a minute ago   ENV LIBBEAT_MONITORING_CGROUPS_HIERARCHY_OVE…   0B        buildkit.dockerfile.v0
<missing>      About a minute ago   USER 1000                                       0B        buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c useradd -M --uid 1000 --gid 1…   4.42kB    buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c groupadd --gid 1000 filebeat …   1.63kB    buildkit.dockerfile.v0
<missing>      About a minute ago   COPY /usr/share/filebeat/NOTICE.txt /license…   3.28MB    buildkit.dockerfile.v0
<missing>      About a minute ago   COPY /usr/share/filebeat/LICENSE.txt /licens…   13.7kB    buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c mkdir /licenses # buildkit       0B        buildkit.dockerfile.v0
<missing>      About a minute ago   COPY /usr/share/filebeat /usr/share/filebeat…   196MB     buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c chmod 755 /usr/local/bin/dock…   716B      buildkit.dockerfile.v0
<missing>      About a minute ago   COPY docker-entrypoint /usr/local/bin/docker…   716B      buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c set -e ;   TINI_BIN="";   TIN…   23.9kB    buildkit.dockerfile.v0
<missing>      About a minute ago   ENV GODEBUG=madvdontneed=1                      0B        buildkit.dockerfile.v0
<missing>      About a minute ago   ENV PATH=/usr/share/filebeat:/usr/local/sbin…   0B        buildkit.dockerfile.v0
<missing>      About a minute ago   ENV ELASTIC_CONTAINER=true                      0B        buildkit.dockerfile.v0
<missing>      About a minute ago   LABEL org.label-schema.build-date=2024-12-22…   0B        buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c for iter in {1..10}; do      …   56.6MB    buildkit.dockerfile.v0
<missing>      About a minute ago   RUN /bin/sh -c touch /var/mail/ubuntu && cho…   4.47kB    buildkit.dockerfile.v0
<missing>      4 weeks ago          /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B        
<missing>      4 weeks ago          /bin/sh -c #(nop) ADD file:765dfd09ec2ac4870…   101MB     
<missing>      4 weeks ago          /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B        
<missing>      4 weeks ago          /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B        
<missing>      4 weeks ago          /bin/sh -c #(nop)  ARG LAUNCHPAD_BUILD_ARCH     0B        
<missing>      4 weeks ago          /bin/sh -c #(nop)  ARG RELEASE                  0B        

After the change

docker.elastic.co/beats/filebeat 8.17.1 5931ccc2e70c 319MB

docker history

IMAGE          CREATED          CREATED BY                                      SIZE      COMMENT
5931ccc2e70c   52 seconds ago   CMD ["-environment" "container"]                0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   ENTRYPOINT ["/usr/bin/tini" "--" "/usr/local…   0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   WORKDIR /usr/share/filebeat                     0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   ENV LIBBEAT_MONITORING_CGROUPS_HIERARCHY_OVE…   0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   USER 1000                                       0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   RUN /bin/sh -c useradd -M --uid 1000 --gid 1…   333kB     buildkit.dockerfile.v0
<missing>      52 seconds ago   RUN /bin/sh -c groupadd --gid 1000 filebeat …   1.67kB    buildkit.dockerfile.v0
<missing>      52 seconds ago   COPY /usr/share/filebeat/NOTICE.txt /license…   3.28MB    buildkit.dockerfile.v0
<missing>      52 seconds ago   COPY /usr/share/filebeat/LICENSE.txt /licens…   13.7kB    buildkit.dockerfile.v0
<missing>      52 seconds ago   RUN /bin/sh -c mkdir /licenses # buildkit       0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   COPY /usr/share/filebeat /usr/share/filebeat…   196MB     buildkit.dockerfile.v0
<missing>      52 seconds ago   RUN /bin/sh -c chmod 755 /usr/local/bin/dock…   716B      buildkit.dockerfile.v0
<missing>      52 seconds ago   COPY docker-entrypoint /usr/local/bin/docker…   716B      buildkit.dockerfile.v0
<missing>      52 seconds ago   RUN /bin/sh -c set -e ;   TINI_BIN="";   TIN…   23.9kB    buildkit.dockerfile.v0
<missing>      52 seconds ago   ENV GODEBUG=madvdontneed=1                      0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   ENV PATH=/usr/share/filebeat:/usr/local/sbin…   0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   ENV ELASTIC_CONTAINER=true                      0B        buildkit.dockerfile.v0
<missing>      52 seconds ago   LABEL org.label-schema.build-date=2024-12-22…   0B        buildkit.dockerfile.v0
<missing>      53 seconds ago   RUN /bin/sh -c for iter in {1..10}; do      …   53.6MB    buildkit.dockerfile.v0
<missing>      2 months ago     /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B        
<missing>      2 months ago     /bin/sh -c #(nop) ADD file:8537b4db344382b39…   65.7MB    
<missing>      2 months ago     /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B        
<missing>      2 months ago     /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B        
<missing>      2 months ago     /bin/sh -c #(nop)  ARG LAUNCHPAD_BUILD_ARCH     0B        
<missing>      2 months ago     /bin/sh -c #(nop)  ARG RELEASE                  0B        

---

This is an automatic backport of pull request #42144 done by [Mergify](https://mergify.com).

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request has not been merged yet. Could you please review and merge it @rdner? 🙏