[workloads] downlevel workload version improvements

[lewing]

#79299 makes release/7.0 calculate the downlevel versions based on the current PatchVersion, this avoids situations where we forget to bump the version in the stable branches but once we start inserting net8 previews we will have to make sure multiple (net8;net7;net7) versions match here and in dotnet/emsdk with the versions we actually intend to insert.

We should also add tests to the sdk related tests that verify the downlevel versions we see in the sdk are never ahead of what is here

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[lewing]

cc @marcpopMSFT @joeloff

[lewing]

We should at the least make sure our versions are in order prior to preview1