Replace NETCOREAPP compiler directive with NET

[ViktorHofer]

Follow-up on 2616119

Consistently use the NET preprocessor directive
instead of a mix of both NETCOREAPP and NET.

@jkotas this is a straight search&replace of NETCOREAPP->NET and !NETCOREAPP->!NET. Do we want to take this change? The benefit is just consistency without any kind of enforcement so I'm fine with taking or closing this change.

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Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[jkotas]

LGTM based on cursory review. I would be great if you can search for and fix outliers like the ones @sbomer pointed out.

[sbomer]

You may need to update

runtime/src/tools/illink/test/Mono.Linker.Tests/TestCasesRunner/TestCaseCompilationMetadataProvider.cs

Line 79 in 017593d

yield return "NETCOREAPP";

and

runtime/src/coreclr/tools/aot/ILCompiler.Trimming.Tests/TestCasesRunner/TestCaseCompilationMetadataProvider.cs

Line 80 in 017593d

yield return "NETCOREAPP";