refactor: Rename CTransaction::nVersion to version

In order to ensure that the change of nVersion to a uint32_t in the
previous commit has no effect, rename nVersion to version in this commit
so that reviewers can easily spot if a spot was missed or if there is a
check somewhere whose semantics have changed.

test/functional/test_framework/wallet.py

@@ -321,7 +321,7 @@ class MiniWallet:
         tx = CTransaction()
         tx.vin = [CTxIn(COutPoint(int(utxo_to_spend['txid'], 16), utxo_to_spend['vout']), nSequence=seq) for utxo_to_spend, seq in zip(utxos_to_spend, sequence)]
         tx.vout = [CTxOut(amount_per_output, bytearray(self._scriptPubKey)) for _ in range(num_outputs)]
-        tx.nVersion = version
+        tx.version = version
         tx.nLockTime = locktime
 
         self.sign_tx(tx)