Simplify conditional compilation .NET

[Kielek]

There are many places in the code where we needed to change NET60_OR_GREATER to NET80_OR_GREATER. It seems like the intent of the directive in many, if not most of those locations was to use a version of the code that only applied to the .net build target and not the .net framework build target.

Where appropriate, should we change NET80_OR_GREATER to either NET or !NETFRAMEWORK, to make the intent clearer?

Originally posted by @nrcventura in #3796 (review)