doc: Add wallet release notes for 0.21.0

Author: MarcoFalke

doc/release-notes.md

@@ -89,6 +89,12 @@ Wallet
 
 #### Wallet RPC changes
 
+- The `upgradewallet` RPC replaces the `-upgradewallet` command line option.
+  (#15761)
+- The `settxfee` RPC will fail if the fee was set higher than the `-maxtxfee`
+  command line setting. The wallet will already fail to create transactions
+  with fees higher than `-maxtxfee`. (#18467)
+
 GUI changes
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