chore(deps-dev): update gunicorn requirement from ==20.* to >=20,<22

[dependabot[bot]]

Updates the requirements on gunicorn to permit the latest version.

Release notes

Sourced from gunicorn's releases.

Gunicorn 21.2.0 has been released

Gunicorn 21.2.0 has been released. This version fix the issue introduced in the threaded worker.

Changes:

21.2.0 - 2023-07-19
===================
fix thread worker: revert change considering connection as idle .
*** NOTE ***
This is fixing the bad file description error.

1. Documentation is available there: https://docs.gunicorn.org/en/stable/news.html
2. Packages: https://pypi.org/project/gunicorn/

Commits

• ab9c830 bump to 21.2.0
• f5669f6 Merge pull request #3033 from benoitc/fix-gthread
• bc90585 revert change considering connection as idle
• 4e12ebe bump to 21.1.0
• 60b78e3 fix setup.cfg: use new license_files property
• 83c3973 Merge pull request #3030 from benoitc/fix-ghthread
• 86d85cb fix gthread worker
• 547f856 bump 21.0.1: fix doc
• 033dca6 fix doc path to conf.py in readthedocs.yaml
• 1dd24e6 fix .readthedocs.yaml path
• Additional commits viewable in compare view

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[steveoh]

@dependabot rebase

[dependabot[bot]]

Looks like gunicorn is no longer updatable, so this is no longer needed.