Add support for Alpine 3.20 (once released)

[MichaelSimons]

This issue is to track adding support for Alpine 3.20 once it is released. Following their release schedule, it is reasonable to expect it in early May.

Parent issue: https://github.com/dotnet/core/issues/9292

ImageBuilder Tasks

• [ ] Ensure that the ImageBuilder supports the new distro version in the code to generate the correct README display name from the version specified in the manifest

Nightly Branch Tasks

1. • [ ] Copy the Dockerfiles of the most recent version of that distro (or author new ones for an entirely new distro) and place them in a version-specific folder under their respective variants (runtime-deps, runtime, aspnet, sdk). Note: not all variants have a corresponding runtime-deps image.
2. • [ ] Modify the Dockerfiles as appropriate for any specific changes related to the new distro version
3. • [ ] Update manifest.json to reference the new set of Dockerfiles with the appropriate tags
     • [ ] Move any distro-specific floating tags to the newer version (e.g. 6.0-alpine)
4. • [ ] Update the test data to include the new distro version
5. • [ ] Update the tags metadata templates to include the new distro version
6. • [ ] Run the command to update the READMEs: .\eng\readme-templates\Get-GeneratedReadmes.ps1
7. • [ ] Run the command to update the image size baseline file: .\tests\performance\Validate-ImageSize.ps1 -UpdateBaselines
8. • [ ] Inspect generated changes for correctness
9. • [ ] Consider whether sample Dockerfiles should be authored if this is a new distro and them to the samples
10. • [ ] Run the command to build and test your changes: .build-and-test.ps1 -OS <os>
11. • [ ] Commit generated changes
12. • [ ] Create PR
13. • [ ] Get PR signoff
14. • [ ] Merge PR to nightly branch
15. • [ ] Wait for automatically queued CI build to finish on dotnet-docker-nightly pipeline (internal MSFT link)
16. • [ ] Confirm READMEs have been updated in Docker Hub

Main Branch Tasks

1. • [ ] After the product teams have signed off on the new distro, merge these changes to the main branch as part of the release process for the next .NET release
     • [ ] For Alpine, follow the policy to only update the floating alpine tag one month after the release of the new Alpine version. Since nightly will already have the floating tag updated, the initial release of the new Alpine version to main will need to account for this and keep it set to the older version for one month.
2. • [ ] Create an announcement (example: Alpine 3.10) unless the new distro is added only for pre-release versions in which the announcement would be incorporated in the pre-release notes.

[lbussell]

[Triage] 3.20 support was already added to the .NET 6.0 RID graph as part of https://github.com/dotnet/runtime/pull/99560.

[lbussell]

Alpine 3.20 images are now available: https://github.com/dotnet/dotnet-docker/discussions/5556