build(deps): bump gunicorn from 20.1.0 to 21.2.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 21.2.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.1.0...21.2.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,7 +1,7 @@
 beautifulsoup4==4.12.2
 bleach==6.0.0
 datacite==1.1.3
-gunicorn==20.1.0
+gunicorn==21.2.0
 hepdata-converter-ws-client==0.2.2
 hepdata-validator==0.3.3
 invenio-access==2.0.0     # Indirect (needed by invenio-admin)