Admin Menu: Fix multiple active menu highlights on mobile.

[itzmekhokan]

Use :focus-visible for admin menu focus indicators so touch taps do not leave a stale active appearance on previously selected items. Clear the responsive selected state when another top-level menu is opened or when navigating via a submenu link. Limit admin menu view transitions to the current menu item to avoid overlapping active styles during navigation.

Fixes #65280.

Trac ticket: https://core.trac.wordpress.org/ticket/65280

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