Preprocessor symbol 'NET' should be automatically defined when building for .NET 5 or higher

[terrajobst]

Relevant portion from the spec:

• We continue to define NETCOREAPP for backwards compatibility with existing #if code.
• Moving forward we'll use NET as the versionless symbol (for .NET Framework we used NETFRAMEWORK, so no conflict there).
• We'll follow the rule-based creation in SDK-style projects which takes the friendly TFM name without the OS flavor, makes it upper-case and replaces special characters with an underscore.
• For OS flavors we'll do the same, i.e. create versionless as well as version specific symbols.

[sfoslund]

@terrajobst is there a repro for a scenario where you're not seeing these symbols produced? They should have been added with #11635 and #12124.

[marcpopMSFT]

That's on me as I swear I tested it and it didn't work but I tried it again now and see NET is defined so I think this can be closed.

[marcpopMSFT]

BTW, confirmed working in RC1

[sfoslund]

Closing per Marc's comment

[Maxyeah]

What about the "OS flavored" preprocessors like NET6_0_WINDOWS. I don´t get these to work. When will these implemented?