Accessibility: Media: Add an `aria-pressed` attribute to active button within a button group.

The `aria-pressed` attribute communicates semantically the "active" state of buttons that otherwise only look "active".

Fixes #48355.

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git-svn-id: https://core.svn.wordpress.org/trunk@46548 1a063a9b-81f0-0310-95a4-ce76da25c4cd