Docs: Correct `@since` version for `available_permalink_structure_tags` filter.

Follow-up to [41182].

Props tiagohillebrandt, sabernhardt.
Fixes #42624.
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git-svn-id: https://core.svn.wordpress.org/trunk@47667 1a063a9b-81f0-0310-95a4-ce76da25c4cd