Gum styling update 2025 8

[vchelaru]

This change does the following:

1. Updates all Initialize calls to use the "V2" styling. This is the recommended approach for current Gum projects.
2. Updates screenshots in tutorial 20 to use the new V2 styling appearances.
3. Updates namespaces from MonoGameGum.Forms -> Gum.Forms to avoid warnings and future removal of old namespace.
4. Updated link to new Gum Upgrading root page.
5. Updates tutorial 21 to use the new ButtonVisual, cutting out ~10 lines of code and simplifying styling.

[AristurtleDev]

Couple of minor grammatical changes.

[SimonDarksideJ]

Some review comments @vchelaru which I agree with, if you can update please.