Add release notes for keypool flush changes

doc/release-notes-23093.md

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+Notable changes
+===============
+
+Updated RPCs
+------------
+
+- `upgradewallet` will now automatically flush the keypool if upgrading
+from a non-HD wallet to an HD wallet, to immediately start using the
+newly-generated HD keys.
+- a new RPC `newkeypool` has been added, which will flush (entirely
+clear and refill) the keypool.