Remove .NET 3.5 and 4.0 targets

[rprouse]

Fixes #3708
Fixes #3758

• Removes .NET 3.5 and 4.0 targets from the build scripts, nuspecs and the project files
• Remove .NET 3.5 and 4.0 pragmas and code
• Remove TASK_PARALLEL_LIBRARY_API pragma as it is now supported on all platforms
• Remove NET45 pragmas that are no longer needed

[rprouse]

Sorry for such a large review guys. At least it is mainly just code deletion. 😺

[stevenaw]

Phew, that was a big review. Thanks for putting this all together @rprouse
Most everything made sense, but I noticed a few things while searching the codebase for 3.5 on your branch that might still be lingering. GitHub doesn't make it easy (to my knowledge) to comment on lines too far from a changed line, but I think there's still a few things that could be removed in RuntimeFramework and PlatformDetectionTests:

• nunit/src/NUnitFramework/framework/Internal/RuntimeFramework.cs

  Lines 113 to 114 in ad49f27

  	major = 3;
  	minor = 5;

• nunit/src/NUnitFramework/tests/Internal/PlatformDetectionTests.cs

  Line 274 in ad49f27

  public void DetectNet35()

EDIT: Though not sure if we care about those within this PR if this is focused just on the compile-time stuff

[rprouse]

Thanks for a thorough review @stevenaw, I really appreciate it. I've resolved all of your suggestions except the DetectNet35() test. There are many OS and runtime tests in that class that are no longer supported by NUnit, so I plan to do a full pass at the Platform attribute and related classes soon and clean them up in a separate PR.

[jnm2]

Feels good!

[stevenaw]

Thanks @rprouse for the quick response. That makes sense about tackling Platform separately

[rprouse]

Damn, the latest builds started failing because .NET 5.0 is not installed on Linux or OSX on Azure. We just removed the install because newer versions were installed on those platforms! 👿

[jnm2]

@rprouse I've had issues with Azure Pipelines images where simply installing an older runtime causes the preinstalled runtimes to no longer be found, so once I install one, I have to install all. Could that be it?

[rprouse]

@jnm2 we removed the install of 5.0 a few weeks ago for just that reason, a newer version came pre-installed on the build images. Now it appears that it isn't installed anymore!