Update usage of Ubuntu 20.04 buildtools containers

[mthalman]

Ubuntu 20.04 buildtools containers are EOL: dotnet/dotnet-buildtools-prereqs-docker#1449

References to these images need to be upgraded to a supported version:

• diagnostics/eng/pipelines/pipeline-resources.yml

  Line 89 in a083d65

  image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-20.04

[steveisok]

We should test bumping to 22.04 and have copilot give this a spin.