Replace usage of out-of-support TFM compiler directives

[ViktorHofer]

... and directives for TFMs that we don't target anymore in main.

Replaces the following directives with NET or !NET:

• #if NETCOREAPP2*
• #if !NETCOREAPP2*
• #if NETCOREAPP3*
• #if !NETCOREAPP3*
• #if NET5*
• #if !NET5*
• #if NET6*
• #if !NET6*
• #if NET7*
• #if !NET7*

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[jkotas]

#if NETCOREAPP2*

Is there a reason why you have not replaced #if NETCOREAPP?

[jkotas]

Some of the test failures look related to the change.

[ViktorHofer]

Is there a reason why you have not replaced #if NETCOREAPP?

Yeah, 300+ hits. i wanted to keep this scoped on the other directives. I can send another PR to then replace NETCOREAPP with NET if we want to go that direction. Another reason for not doing this as part of this PR is that we aren't consistent between using NETCOREAPP vs NET in general, i.e. TFM properties still use NetCoreAppX instead of just NetX and I didn't want to submit a questionable change.

[carlossanlop]

Do you mind sharing why this change is being made? Why did the old directives lose their value?

Assuming the CI is all good, LGTM.

[ViktorHofer]

Do you mind sharing why this change is being made? Why did the old directives lose their value?

The main branch doesn't benefit from the distinction between NETCOREAPP2 vs NETCOREAPP3 vs NET5 vs NET6 vs NET7 as non of those TFMs are built anymore. Most of them are already out-of-support or soon will be. From a code point perspective, differentiating between .NET Framework, modern .NET and specific modern .NET versions that we target and build in main is sufficient.

Or to rephrase, NETCOREAPP2 and similar compiler directives are things from the past that we don't need in our main source anymore.