Make sure aria-disabled buttons use a consistent focus style

[afercia]

Splitting this out from #66038.
See comment at ​https://github.com/WordPress/wordpress-develop/pull/13390/#issuecomment-6061122370

Now that Core is going to use more and more the pattern already used on Gutenberg where disabled controls use aria-disabled="true" instead of a disabled attribute, the styles that come from buttons.css, colors.css, and potentially other stylesheets should be adapted to ensure the aria-disabled buttons get a consistent focus style.

Reminder: using aria-disabled="true":

• Helps making controle discoverable, for better general usability.
• Helps preventing focus losses when controls are disabled, as they are still focusable.