Remove the `blocklyFocused` CSS class and use the `focused` CSS selector instead

[BeksOmega]

1. Remove the blocklyFocused class from being assigned in menu.ts.
2. Remove the blocklyFocused class from being assigned in dropdowndiv.ts.
3. Change the styles in css.ts to use the :focus css pseudo class.

This change should be made against the rc/v12.0.0 branch. This should be marked as a breaking change.

To work on this issue, comment below asking to be assigned.

[Skye967]

Can I get assigned to this?

[BeksOmega]

Go for it @Skye967 =) Let me know if you have any questions!

[BeksOmega]

Closed by #8360