bump: ubuntu runners version before deprecation

[dduzgun-security]

Description

The Ubuntu 20.04 image is slated for EOL in April 2025. As part of GitHub’s official N-1 OS support policy, GitHub-hosted Ubuntu 20.04 runners will be decommissioned and unavailable for use after April 1, 2025.

Testing & Reproduction steps

Links

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[Juanadelacuesta]

Looks good, Im not sure the changelog is necessary :)

[tgross]

@dduzgun-security we can't do this until we've sorted out the issue it'll create with our glibc bindings. We have to support quite old versions of RHEL. The plan is to run the build.yml in a hermetic build environment so we can avoid the revs imposed by GitHub and ProdSec.

See https://github.com/hashicorp/nomad-enterprise/issues/1109