Drop net45, net6, Fable3 from F#+ 2 release

[wallymathieu]

In order to simplify the release process, we should drop the target frameworks net45, net6 in the release. Also remove check for Fable3 for F#+ 2. The main reason for this is that there is an Issue with consuming DLLs built with .NET7 from .NET6.

[wallymathieu]

We have dropped net45 from the v1 release track.