UseStableVersions property should be eliminated

[crummel]

We should be able to use checked-in metadata to determine whether to build a given repo as stabilized or unstabilized. Previously when I tried this I ran into issues with WriteSourceRepoProperties not getting the stable/unstable metadata correctly.

[dagood]

We should be able to use checked-in metadata to determine

Since ProdCon v2/Arcade/Darc (3.0+), I think we shouldn't even need to determine it... the value is checked in, so using a default build command should create the right stabilization.

https://github.com/dotnet/corefx/blob/c68bc274ce95cb5eda143223942d6fd86a547a0d/eng/Versions.props#L14
https://github.com/dotnet/installer/blob/v3.1.201/src/redist/targets/Versions.targets#L11

In ProdCon v1, we had to know it because an orchestrator passed in a value, which could be changed without any source code changes. It was literally impossible to know based on the source code. 😛 I expect we'll always need to pass this in manually in release/2.1.

In ProdCon v2, there's no more orchestrator. As far as this kind of build param goes, it's deterministic.

[dagood]

Triage:
@dseefeld ran into stabilization property issues in 5.0, and I think we should probably do this there. We don't think it's necessary in 3.1 because things should be stable there.