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/mempool_accept.py

@@ -287,7 +287,7 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
 
         self.log.info('Some nonstandard transactions')
         tx = tx_from_hex(raw_tx_reference)
-        tx.nVersion = 4  # A version currently non-standard
+        tx.version = 4  # A version currently non-standard
         self.check_mempool_result(
             result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'version'}],
             rawtxs=[tx.serialize().hex()],