Merge #18733: doc: Add wallet release notes for 0.21.0

facaefa doc: Add wallet release notes for 0.21.0 (MarcoFalke)
faa4243 Add release notes skeleton, so that notes can be filled easier (MarcoFalke)

Pull request description:

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Tree-SHA512: a17ab86e422ca3d3e53deffa7fecf09cdd9b816588deeded3a15e80a1c268ff1e8b56a0e052a417f1a091872099cd3d2b89993d4773a86516b0bdef880a949a0

Author: MarcoFalke

doc/release-notes.md

@@ -62,6 +62,48 @@ distribution provides binaries for the RISC-V platform.
 Notable changes
 ===============
 
+P2P and network changes
+-----------------------
+
+Updated RPCs
+------------
+
+Changes to Wallet or GUI related RPCs can be found in the GUI or Wallet section below.
+
+New RPCs
+--------
+
+Build System
+------------
+
+Updated settings
+----------------
+
+Changes to Wallet or GUI related settings can be found in the GUI or Wallet  section below.
+
+New settings
+------------
+
+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
+-----------
+
+Low-level changes
+=================
+
+Tests
+-----
+
 Credits
 =======