add changelog

changelog/unreleased/fix-plugins-go1.21 copy.md

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+Enhancement: Expire cached users and groups entries
+
+Entries in the rest user and group drivers do not expire.
+This means that old users/groups that have been deleted are still in cache.
+Now an expiration of `fetch interval + 1` hours has been set.
+
+https://github.com/cs3org/reva/pull/4121