The Source element in Version.Details.xml sometimes gets incorrectly updated by automation

### Pull request

https://github.com/dotnet/templating/blob/release/10.0.2xx/eng/Version.Details.xml#L3

### Type of problem

Codeflow PRs not getting created

### Description of the issue

See the above link. When using automation like the inter-branch-flow or just a manual branch merge, people have to pick ours vs theirs for the Source element in eng/Version.Details.xml. In at least the example of dotnet/templating's release/10.0.2xx branch, the wrong one was picked.

I think this will happen again and again as choosing the right one to pick is not obvious. We probably want some sort of validation in repo CI, not just for this case but also for the synchronicity of Version.Details.xml with Version.Details.props.

Additionally, as discussed offline, it would be good to remove this metadata entirely from source by leveraging mechanics like git notes.

cc @mmitche