Improve the maintainability of upgrading the distros/versions used in various CI/testing legs

[MichaelSimons]

This is not easy to do depending on which distro is being updated. The worst is updating the distro leg used in PR validation (currently the lowest centos version). This leg/artifacts are referenced across several places in the code base. At a minimum using a shared variables template would help solve the majority of references. There are some outliers, for example the prep script references the default rid to retrieve the artifacts for.

Example illustrating the places that need updating - dotnet/installer#19119

[mthalman]

Part of this work should include updating the SDK and artifacts tarballs that are stored in blob storage so that they are version-less with respect to the distro. Today they have the full RID:

• dotnet-sdk-9.0.100-preview.4.24222.1-centos.9-x64.tar.gz
• dotnet-sdk-9.0.100-preview.4.24223.1-alpine.3.19-x64.tar.gz

Ideally we could even get rid of the distro:

• dotnet-sdk-9.0.100-preview.4.24222.1-linux-x64.tar.gz
• dotnet-sdk-9.0.100-preview.4.24222.1-linux-musl-x64.tar.gz

Or at the very least discard the distro version:

• dotnet-sdk-9.0.100-preview.4.24222.1-centos-x64.tar.gz
• dotnet-sdk-9.0.100-preview.4.24223.1-alpine-x64.tar.gz