Consider whether to keep the N-1 Alpine build leg

[mthalman]

The N-1 Alpine build leg is problematic in the transition between major .NET versions because it needs to be bootstrapped. It requires that there be an artifacts tarball for the N-1 build based on the new version of .NET. But that can't exist because there's not a version of the Alpine leg that exists that is based on the current SDK (we use the current SDK for the latest Alpine version and the previous SDK for the previous Alpine version). So the only way to get artifacts are to manually run a build that explicitly targets the previous Alpine version. We should consider whether this hassle is worth it.

Related to #4605

[MichaelSimons]

[Triage] We feel there is not ROI for the alpine leg that build with the n-1 alpine artifacts. The alpine leg should be updated to build with the MSFT sdk and prep.

[ellahathaway]

Will close once the release pipeline PR is merged.

Edit: PR is merged, closing this issue.