Update Roslyn to 4.12 for new generators

[Sergio0694]

This PR switches the new analyzers in the MVVM Toolkit to the preview of Roslyn 4.12 (not stable just yet).

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

[milesdream]

I stiil wait for something to have a small Letter as the public Property

at example
[ObservableProperty]
private bool _bReady ;

produces a public Property
public bool bReady;

its allready funny if I get Error CS0576 A partial method may not have multiple defining declarations

if I do

[ObservableProperty]
private bool _bNothing;

[ObservableProperty]
private bool bNothing;