Fixup build to clean state

[mgravell]

This updates #431:

• updating packages
• enables API tracking
• cleans up remaining build output
• applies modern C# standards
• changes test matrix from net6+net7+net8 to net8+net9 - removed EOL (and not usable with current tooling)

Note that this will involve updating the CI so that the build happens with SDK 9, proposed here

Note there are some large machine-automated refactors in here; that's why API tracking was enabled: to prove that they haven't broken the API.

[uglide]

Awesome, thanks for putting your effort into this clean up!