consensus: Store transaction nVersion as uint32_t

Given that the use of a transaction's nVersion is always as an unsigned
int, it doesn't make sense to store it as signed and then cast it to
unsigned.

src/script/interpreter.cpp

@@ -1743,7 +1743,7 @@ bool GenericTransactionSignatureChecker<T>::CheckSequence(const CScriptNum& nSeq
 
     // Fail if the transaction's version number is not set high
     // enough to trigger BIP 68 rules.
-    if (static_cast<uint32_t>(txTo->nVersion) < 2)
+    if (txTo->nVersion < 2)
         return false;
 
     // Sequence numbers with their most significant bit set are not