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/rpc_packages.py

@@ -394,7 +394,7 @@ class RPCPackagesTest(BitcoinTestFramework):
         peer = node.add_p2p_connection(P2PTxInvStore())
         txs = self.wallet.create_self_transfer_chain(chain_length=2)
         bad_child = tx_from_hex(txs[1]["hex"])
-        bad_child.nVersion = 0xffffffff
+        bad_child.version = 0xffffffff
         hex_partial_acceptance = [txs[0]["hex"], bad_child.serialize().hex()]
         res = node.submitpackage(hex_partial_acceptance)
         assert_equal(res["package_msg"], "transaction failed")