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.

In %18.9 (removal milestone)

• Drop builds for Ubu 20.04.
• Drop related builder images: https://gitlab.com/gitlab-org/gitlab-omnibus-builder.
• Move Ubu 20 to deprecated OSes.
• Update the install page. This needs an issue in https://gitlab.com/gitlab-com/marketing/digital-experience/buyer-experience.