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:

• dotnet-monitor/eng/pipelines/templates/pipeline-template.yml

  Line 38 in a7fe625

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

• dotnet-monitor/eng/helix/Helix.proj

  Line 82 in a7fe625

  <HelixAvailableTargetQueue Include="ubuntu.2204.amd64$(QueueSuffix)@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-20.04-helix-amd64"

These changes also need to be backported to the servicing branches.