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:

• wcf/azure-pipelines-arcade-PR.yml

  Line 44 in faddd7a

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