Merge bitcoin/bitcoin#24494: wallet: generate random change target for each tx for better privacy

9053f64fcb [doc] release notes for random change target (glozow)
46f2fed6c5 [wallet] remove MIN_CHANGE (glozow)
a44236addd [wallet] randomly generate change targets (glozow)
1e52e6bd0a refactor coin selection for parameterizable change target (glozow)

Pull request description:

  Closes #24458 - the wallet always chooses 1 million sats as its change target, making it easier to fingerprint transactions created by the Core wallet. Instead of using a fixed value, choose one randomly each time (within a range). Using 50ksat (around $20) as the lower bound and `min(1 million sat, 2 * average payment value)` as the upper bound.
  RFC: If the payment is <25ksat, this doesn't work, so we're using the range (payment amount, 50ksat) instead.

ACKs for top commit:
  achow101:
    ACK 9053f64fcb
  Xekyo:
    reACK 9053f64fcb

Tree-SHA512: 45ce5d064697065549473347648e29935733f3deffc71a6ab995449431f60302d1f9911a0994dfdb960b48c48b5d8859f168b396ff2a62db67d535a7db041d35

test/functional/wallet_bumpfee.py

@@ -442,7 +442,9 @@ def test_watchonly_psbt(self, peer_node, rbf_node, dest_address):
     self.generate(peer_node, 1)
 
     # Create single-input PSBT for transaction to be bumped
-    psbt = watcher.walletcreatefundedpsbt([], {dest_address: 0.0005}, 0, {"fee_rate": 1}, True)['psbt']
+    # Ensure the payment amount + change can be fully funded using one of the 0.001BTC inputs.
+    psbt = watcher.walletcreatefundedpsbt([watcher.listunspent()[0]], {dest_address: 0.0005}, 0,
+            {"fee_rate": 1, "add_inputs": False}, True)['psbt']
     psbt_signed = signer.walletprocesspsbt(psbt=psbt, sign=True, sighashtype="ALL", bip32derivs=True)
     psbt_final = watcher.finalizepsbt(psbt_signed["psbt"])
     original_txid = watcher.sendrawtransaction(psbt_final["hex"])