test: rename CTransaction .rehash()/.hash -> .txid_hex for consistency

Note that we unfortunately can't use a scripted diff here, as the same
property and method name is also used for `CBlockHeader`/`CBlock` instances.

test/functional/feature_utxo_set_hash.py

@@ -55,7 +55,7 @@ class UTXOSetHashTest(BitcoinTestFramework):
                     if (coinbase and n > 0):
                         continue
 
-                    data = COutPoint(int(tx.rehash(), 16), n).serialize()
+                    data = COutPoint(int(tx.txid_hex, 16), n).serialize()
                     data += (height * 2 + coinbase).to_bytes(4, "little")
                     data += tx_out.serialize()