Change accordions in the customizer and the navigation menus to make proper usage of accordion markup patterns. This includes adding missing `:focus` states, using a `button` element to control tabbing and interaction, instead of the heading elements, and removing instructional text for screen reader users that was used to compensate for the incorrect markup pattern.
Props afercia, rishishah, kushang78, rcreators, krupajnanda, hmbashar, joedolson.
Fixes#42002.
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git-svn-id: http://core.svn.wordpress.org/trunk@58616 1a063a9b-81f0-0310-95a4-ce76da25c4cd
