Remove hardcoded TFMs and replace powershell script with MSBuild projects

Two powershell scripts currently hardcode the TFM value:

- https://github.com/dotnet/format/blob/0b968051beac5d7c1a62a52aee7fbbbb47dc1f47/eng/format-verifier.ps1#L76
- https://github.com/dotnet/format/blob/0b968051beac5d7c1a62a52aee7fbbbb47dc1f47/eng/perf.ps1#L48

While that wasn't a problem in the past, with now floating TFM versions defined by Arcade (`$(NetCurrent)`), the goal is to upgrade repositories to the next version of .NET more easily.

Ideally the two scripts would be replaced by simple msbuild projects (using the `Microsoft.Build.NoTargets` SDK) that achieve the exact same result (potentially with even less code). MSBuild has access to the entire repo infrastructure, including the `NetCurrent` proprerty and hence those hardcoded entries would go away.