Update the base image to jammy-20220421 and apt-cacher-ng to 3.7.4

[gromain]

This PR updates the apt-cacher-ng image to the latest version of both apt-cacher-ng (3.7.4 at this time) and the base image to focal-20220113.

Also, as a simplification of the versioning, the version now references the base image version date, and not the actual build date.

[gromain]

Converted this PR to a draft for now.

apt-cacher-ng 3.7.4 is not available for Ubuntu focal.
Jammy is going to be the next LTS scheduled for release late april.

Until then this will stay a draft and will be updated once the final stable images for jammy are released.

[gromain]

With the release of jammy stable, this PR is now ready.
Commits have been squashed and cleaned up.
Version is consistent across all occurrences and references the current base image build date (version string is 3.7.4-20220421).

[gromain]

Image is verified to work with gromain/apt-cacher-ng:autobuild.

Also, should close #31.

[gromain]

Any needed changes on my side? Did you have time to test @sameersbn ?

[gromain]

@sameersbn would you accept merging this? Is there anything needed on my side?

[guenhter]

@sameersbn Is there anything what blocks this MR from being merged?

[sameersbn]

thanks for the contribution

[gromain]

My pleasure! Thanks for merging!