Update build containers

[ericstj]

It looks like we're using ubuntu 18.04 build containers which are no longer built / supported.

We should update to some version that's in support and ensure it doesn't result in changing our product support pre-requisites.

Seems that .NET 8.0 is using azurelinux here, maybe we could do the same. We'd need to build new containers in https://github.com/dotnet/dotnet-buildtools-prereqs-docker

We should do the same for both official and PR validation builds.

@michaelgsharp @MichaelSimons