Adds a z-index to `.media-menu .media-menu-item:focus` to fix the "Upload files" tab's outline on the side next to the other tab.
Follow-up to [57553], [46363].
Props wildworks, antpb, huzaifaalmesbah, krupajnanda, nhrrob, oglekler, rajinsharwar, sabernhardt.
Fixes#60632.
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