Remove Alpine 3.20 Dockerfiles

[lbussell]

In the January 2025 servicing release, Alpine 3.21 container images were published. In the February 2025 servicing release, all Alpine floating tags were updated to target Alpine 3.21 instead of Alpine 3.20 according to our tagging policy.

Per the .NET Docker platform support policy, Alpine 3.20 images will no longer be maintained starting on May 13, 2025. This issue tracks removing those Dockerfiles.