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](https://github.com/benoitc/gunicorn/compare/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] <support@github.com>

requirements.txt

@@ -40,7 +40,7 @@ google-cloud-tasks==2.13.2
 googleapis-common-protos==1.59.1
 grpc-google-iam-v1==0.12.6
 grpcio==1.56.0
-gunicorn==20.1.0
+gunicorn==21.2.0
 html2text==2020.1.16
 html5lib==1.1
 humanfriendly==10.0