rpc: make sure `gettxoutsetinfo` hash has txids

The key (transaction id for the following outputs) should be serialized
to the HashWriter.

This is a problem as it means different transactions in the same
position with the same outputs will potentially result in the same hash.

Fixes primary concern of #7758.

src/rpc/blockchain.cpp

@@ -467,6 +467,7 @@ static bool GetUTXOStats(CCoinsView *view, CCoinsStats &stats)
         CCoins coins;
         if (pcursor->GetKey(key) && pcursor->GetValue(coins)) {
             stats.nTransactions++;
+            ss << key;
             for (unsigned int i=0; i<coins.vout.size(); i++) {
                 const CTxOut &out = coins.vout[i];
                 if (!out.IsNull()) {