Deprecate Ubuntu 20.04

Ubuntu 20.04 will be EOL in April 2025. We should deprecate the builds to give folks a heads up to upgrade to Ubuntu 22.04.

Things to discuss:

1. I expect Ubuntu 20 to still have a high adoption based on #8447 (comment 1831581780). Do we have any up to date numbers of this?
2. Ubuntu offers extended support for Pro. Our past Ubuntu deprecation followed the end standard support. Is there any reason to change this?
3. Should we announce the removal more then 3 months in advance this time (proposal: 6 months)?
4. Do we use Ubuntu 20 packages on any internal instance or .com or Dedicated?
5. FIPS users need #8908 (closed) to upgrade.

Acceptance Criteria

In %17.9 / %17.10

• Agree on a deprecation timeline and merge the deprecation announcement.
• Unblock upgrades for FIPS users: Provide FIPS packages for Ubuntu 22.04 (#8908 - closed).

Between deprecation announcement and removal

• Check internal usage and migrate systems before removal: Coordinate internal Ubuntu 20.04 migration befo... (#9750 - closed)

In %18.9 (removal milestone)

• Drop builds for Ubuntu 20.04: Drop Ubuntu 20.04 package builds from omnibus-g... (#9751 - closed)
• Drop related builder images: Remove Ubuntu 20.04 build container images from... (gitlab-omnibus-builder#113)
• Move Ubuntu 20 to deprecated OSes: Move Ubuntu 20.04 to deprecated OSes in support... (#9752 - closed)
• Update the install page: gitlab-com/marketing/digital-experience/about-gitlab-com#1455 (closed)