Admin: A11y: Fix visibility of pressed button state.

Increase the style contrast between pressed and un-pressed button states by using enhanced box-shadow to differentiate.

Props wildworks, mukesh27, shailu25, maulikmakwana2008, huzaifaalmesbah, sabernhardt, khushi1501, ozgursar, noruzzaman, gaisma22, sirlouen, joedolson.
Fixes #64856.

git-svn-id: https://develop.svn.wordpress.org/trunk@62106 602fd350-edb4-49c9-b593-d223f7449a82