Remove net 6.0 targets

[stevejgordon]

• Removes unsupported net6.0 targets from libraries and updates build and test projects accordingly.
• Updates compiler preprocessor directives.
• Updates a few critical dependencies to ensure the build and CI processes continue functioning.
• Refactors StartupHookTests to ensure all dotnet commands run on 8.0.404 SDK. This can be updated and automated as part of upgrading to the .NET 9 SDK in a future PR. Additional logging is added when debugging test failures locally.

Closes #2489

[stevejgordon]

Lgtm, one thing that comes to mind is to double check all our .dotnet-tools.json dependencies run on .net 8.0.

We might get bit by that during release.

Starting with .net 9 we can control this strict sdk pinning for running dotnet tools by specifying it's safe to run this tool on a later sdk (99.99÷) of the time.

Good shout! assembly-differ has a specific net8.0 target, so I think that's fine, and minver-cli has the RollForward MSBuild property. As that's been working on the .NET 8 SDK already, I think that should be ok.