Remove XCopyMSBuild and reenable full framework C++ tests

[sfoslund]

Since #12202 leveraged a new MSBuild feature, we needed to use XCopyMSBuild to get the full framework CI legs working. This change also required several others which should be reverted:

• Disabled full framework C++ tests, which rely on props and targets that are not available with this package.
• Disabled F# tests which rely on props and targets that are not available with this package (see Adding default ProduceReferenceAssembly value for F# #13085 (comment))
• Reverted Updating target framework version parsing length #12812, which relies on a version of MSBuild that was not yet inserted.