Target .NET 8, #928

[paulirwin]

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Adds the .NET 8 target.

Fixes #928

Description

This PR adds the .NET 8 target. Due to a conflict between extension methods in .NET 8 and J2N, we added new methods to Collections to create J2N read-only collections instead of using the extension method. The unit tests will use the net8.0 targets when run under .NET 8 now, instead of net6.0.

Additionally, this resolves a warning when running the .NET 5 tests, and ensures the _artifacts/NuGetPackages folder is created in the build script before restore, which is helpful when you add that folder as a local path in your NuGet sources for testing purposes.

[NightOwl888]

Thanks for the PR.

The changes look good.

This brings us from 93% to 95% of the allowed 10GB on Azure Pipelines, so we are good for now. After #981, this target only brought us from the original distribution size of 127MB up to 130MB.

There was a prior change that hard coded the net6.0 target framework in NativeFSLockFactory which triggered a build warning after the upgrade, which I have addressed in a way that is tolerant of future target framework changes.