Clean up preprocessor directives #577

We currently have NET7_0 directives in the code base, these should be NET7 or greater rather than a specific version.

#if NET7_0

See preprocessor-directives

We have had trouble since NET?_OR_GREATER is not defined. Note the workaround in checking if not net45 et.c..

This is not a bug yet, since we have not yet released a version of F#+ compiled against a version greater than net6 (due to breaking changes in the F# compilation of assemblies).

See for instance:

#if !NET45 && !NETSTANDARD2_0 && !FABLE_COMPILER