Eliminate maintenance issues with pinning the TargetingPackVersion in SBRP

The TargetingPackVersion that pins the TargetingPackVersion for each tfm needs to be updated every release.  It feels plausible to make this logic version agnostic to eliminate the maintenance issue.  It typically breaks/blocks builds/PRs and needs to be fixed quickly.

Example: https://github.com/dotnet/source-build/issues/4850