Stop publishing Ubuntu 20.04 ("Focal") images by May 2025

[lbussell]

Ubuntu 20.04 reaches end of standard support in May 2025. It will stop receiving critical security patches at that time.

Reference: https://ubuntu.com/about/release-cycle

[mthalman]

[Triage]
@siramvikram - There should be a primary issue for this logged in the dotnet/core repo. Can you take care of that please?

@richlander - The issue dotnet/core#9638 doesn't have this version listed for outgoing.

[MichaelSimons]

We will want to ensure tracking issues are logged for all pipelines still using the Ubuntu 20.04 images.

Examples:
https://github.com/dotnet/runtime/blob/release/8.0/eng/pipelines/libraries/helix-queues-setup.yml
https://github.com/dotnet/runtime/blob/release/8.0/eng/pipelines/coreclr/templates/helix-queues-setup.yml

[MichaelSimons]

Related: dotnet/core#9861