text: Title: [MNT]: ci: ubuntu-20.04 GitHub Actions runner will soon be unmaintained
  Body: ### Summary
  
  The `ubuntu-20.04` GitHub Actions runner image, currently [used in the `tests.yml` workflow](https://github.com/matplotlib/matplotlib/blob/3edda656dc211497de93b8c5d642f0f29c96a33a/.github/workflows/tests.yml#L60) will soon be unsupported, as notified at: https://github.com/actions/runner-images/issues/11101
  
  ### Proposed fix
  
  Ubuntu 20.04 itself is a long-term-support release, however it is also nearing the end of that support cycle, and will no longer be supported by Canonical from May 2025: https://ubuntu.com/20-04
  
  I'd suggest removing the `ubuntu-20.04` jobs from the `tests.yml` workflows at the end of this month (March 2025).