[doc] Add release note for adjusted time removal

Co-authored-by: Antoine Riard <antoine.riard@gmail.com>

doc/release-notes-28956.md

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+Adjusted time removal
+=====================
+
+The concept of adjusted time (sum of a node's local time and the median time
+offset of its first 199 outbound connections) is removed from consensus code.
+The warning for a large median time offset (70 minutes or more) is kept and
+still observable by a call to `getblockchaininfo`.
+
+Upon observing this warning, node operators should check that their local time
+and date are correct to make sure they do not fall out of consensus with the
+network.