Re-apply internal staging versions of .NET in sync-internal-release command

[lbussell]

This is the biggest part of https://github.com/dotnet/dotnet-docker-internal/issues/6741. The functionality added in this PR ensures that the internal/release/* branch can be updated with the state of the release/* branch even when the two branches diverge.

I have attempted to make commits reviewable individually.

[lbussell]

The biggest change from the last ~3 commits has to do with the SourceBranch argument. It was used by various update-dependencies commands to determine which base url variables to update. However the new sync command needs to use the SourceBranch option because it operates on two branches.

The "source" branch (or "upstream" branch) is just a property of the repo. For example, our Get-Branch PowerShell function reads the name of the first repo in the manifest to determine the upstream branch. That is passed to the Dockerfile/Readme generation scripts as a variable override. I figured everything would be simpler if we just put it in manifest.versions.json.

The various update-dependencies can now just read it from the manifest variables directly instead of taking it as an argument, which frees up SourceBranch for the sync command to use.

I actually already made this change when working on dotnet/docker-tools#1813 to simplify things. There will need to be a little bit of cleanup in various scripts that still override the branch variable when generating dockerfiles/readmes.

[mthalman]

This shares the same logic as SyncInternalReleaseCommand.ExecuteAsync. It'd be nice if there was one method to format this URL.

[lbussell]

I made sure that SpecificCommand and SyncInternalReleaseCommand both use the extension method. Also centralized the trimming into the options class.