Add .NET 7 TFM to HighPerformance target, switch to C# 11

[Sergio0694]

This PR introduces the following changes:

• Adds the .NET 7 TFM to the HighPerformance package
• Removes the Span<T> trick from the ref-like types, and uses ref fields instead
• Enables trimming analyzers in the HighPerformance package again
• Enables C# 11 for the whole solution
• Also uses some C# 11 sugar where needed

PR Checklist

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