[CI] Align ubuntu version for runners and use fixed version

[bkontur]

Addressing comment: https://github.com/polkadot-fellows/runtimes/pull/637/files#r2012149573

Actually, some workflows use runs-on: ubuntu-22.04, while others use runs-on: ubuntu-latest. Additionally, there are several other places where either ubuntu-22.04 or ubuntu-latest is used.

Open questions

• Decide whether to use ubuntu-22.04 or a higher version like ubuntu-24.04 (Ubuntu Releases)
• Consider an easier configuration method:
  • Using runs-on: ${{ vars.UBUNTU_VERSION }} (but this requires repo admin access to modify variables, which is probably not ideal)
  • Using $GITHUB_ENV and an .env file (though, as I understand, this only works at the step level, not the job level)
  • Using a matrix strategy with several versions (probably not needed)
  • Sticking to manual updates (maybe that's the best option. Anyway, how often do we actually need to change it?)
• Does not require a CHANGELOG entry

[bkchr]

Please use latest everywhere. This is for example the label used by our self hosted runners.