Add Alpine 3.21 images

[lbussell]

Fixes #6095

[lbussell]

I had to make some adjustments to Alpine tests:

• Improved error messages so they're more actionable
• Made it so that the floating tag can be on a specific version instead of always the latest. This is in manifest.versions.json.

Here's an example of the error you get when you make a mistake now:

Microsoft.DotNet.Docker.Tests.StaticTagTests.FloatingAlpineTag_OnLatestVersion (97ms): Error Message: 
      Expected tags to contain a single item matching ^8.0(-preview)?-alpine$.IsMatch(tag) because image DockerfileInfo { Repo = runtime-deps, MajorMinor = 8.0, Os = alpine3.20, Architecture = amd64 } should have an alpine floating tag, but no such item was found.
      Expected tags to contain a single item matching ^8.0(-preview)?-alpine$.IsMatch(tag) because image DockerfileInfo { Repo = runtime-deps, MajorMinor = 8.0, Os = alpine3.20, Architecture = arm32v7 } should have an alpine floating tag, but no such item was found.
      Expected tags to contain a single item matching ^8.0(-preview)?-alpine$.IsMatch(tag) because image DockerfileInfo { Repo = runtime-deps, MajorMinor = 8.0, Os = alpine3.20, Architecture = arm64v8 } should have an alpine floating tag, but no such item was found.