Add .NET 8 TFM to all projects and AOT annotations

This PR adds the .NET 8 TFM to all projects, enables tests on .NET 8, and adds AOT annotations where necessary.

PR Checklist

[X] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
[X] Based off latest main branch of toolkit
[X] PR doesn't include merge commits (always rebase on top of our main, if needed)
[X] Tested code with current supported SDKs
[X] Tests for the changes have been added (for bug fixes / features) (if applicable)
[X] Header has been added to all new source files (run build/UpdateHeaders.bat)
[X] Contains NO breaking changes
[X] Every new API (including internal ones) has full XML docs
[X] Code follows all style conventionsther information that might be helpful to reviewers.