Updates for .NET 8 - Githubissues

This PR makes the following changes:

The .NET 8 SDK is used in CI now
- Various cleanups to address analyzer warnings have been made
- Switched to using the new SDK artifact output instead of customizing it manually
.NET 8 is now including the the testing matrix
- The new macos-14 runner image means we can actually run tests against an arm64 CPU!
.NET 7 has been removed since it's getting close to end of support anyway
Testing packages and CI actions have been updated

libgit2 / libgit2sharp