Ensure Unity publishing works without disrupting other targets

[reeseschultz]

This pull request fixes this problem: #143 (reply in thread).

The tests pass while simultaneously allowing Unity publishing with netstandard2.1.

Now the TargetFramework for projects that otherwise didn't have it is set to net7.0 (see below comment) which will prevent the tests from failing (for that reason). TargetFramework, TargetFrameworks and TargetFrameworkVersion are also now forced in the UnityPublish.sh script as arguments, because it 100% reliably overwrites whereas putting them in Directory.Build.targets apparently didn't. I'm not a fan of MSBuild.

I'm additionally ensuring that any previously published assemblies are deleted before new ones are generated again just to be safe. We don't want old ones to gunk up the directory. And I removed some confusing language about codegen from the shell script. Sorry for all the hassle with this. It's a lot of trial and error wrangling this build system to comply with reason.

Sister PR: genaray/Arch.Extended#25

[reeseschultz]

Amending the above, I went full-circle and removed the TargetFramework property from Arch and every single multi-target project in Arch.Extended (it's back to how it was before the Unity publishing changes). This would have caused an error if I were still trying to overwrite it and related properties inside the Directory.Build.targets file itself, but since I'm now overwriting them as arguments to dotnet msbuild in the shell script, this is perfectly fine. The tests pass (for Arch), the Unity publishing works, and the CI passes as well.

Deep breath.