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

src/qt/coincontroldialog.cpp

@@ -33,7 +33,6 @@
 #include <QTreeWidget>
 
 using wallet::CCoinControl;
-using wallet::MIN_CHANGE;
 
 QList<CAmount> CoinControlDialog::payAmounts;
 bool CoinControlDialog::fSubtractFeeFromAmount = false;
@@ -486,11 +485,10 @@ void CoinControlDialog::updateLabels(CCoinControl& m_coin_control, WalletModel *
             if (!CoinControlDialog::fSubtractFeeFromAmount)
                 nChange -= nPayFee;
 
-            // Never create dust outputs; if we would, just add the dust to the fee.
-            if (nChange > 0 && nChange < MIN_CHANGE)
-            {
+            if (nChange > 0) {
                 // Assumes a p2pkh script size
                 CTxOut txout(nChange, CScript() << std::vector<unsigned char>(24, 0));
+                // Never create dust outputs; if we would, just add the dust to the fee.
                 if (IsDust(txout, model->node().getDustRelayFee()))
                 {
                     nPayFee += nChange;