doc: add release note for fee field deprecation

doc/release-notes.md

@@ -99,6 +99,15 @@ Updated RPCs
   causes the lock to be written persistently to the wallet database. This
   allows UTXOs to remain locked even after node restarts or crashes. (#23065)
 
+- The top-level fee fields `fee`, `modifiedfee`, `ancestorfees` and `descendantfees`
+  returned by RPCs `getmempoolentry`,`getrawmempool(verbose=true)`,
+  `getmempoolancestors(verbose=true)` and `getmempooldescendants(verbose=true)`
+  are deprecated and will be removed in the next major version (use
+  `-deprecated=fees` if needed in this version). The same fee fields can be accessed
+  through the `fees` object in the result. WARNING: deprecated
+  fields `ancestorfees` and `descendantfees` are denominated in sats, whereas all
+  fields in the `fees` object are denominated in BTC. (#22689)
+
 New RPCs
 --------