chore: bump gunicorn to v21

[villebro]

SUMMARY

Gunicorn 21 fixes a few nasty bugs I ran into, namely:

• A deadlock that can choke workers: Gunicorn gthread deadlock benoitc/gunicorn#2917
• An annoying error that can be seen on logs sometimes: ValueError: count must be a positive integer (got 0): Solving issue #2692 benoitc/gunicorn#2773

The latest patch release has been out for a few months now, and seems to work well based on my testing. Also, v 21 adds Support for Python 3.11 which we are mostly supporting now, and want to officially support soon.

The full changelog: https://docs.gunicorn.org/en/latest/news.html

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API