Dump transaction version as an unsigned integer in RPC/TxToUniv

Consensus-wise we already treat it as an unsigned integer (the
only rules around it are in CSV/locktime handling), but changing
the underlying data type means touching consensus code for a
simple cleanup change, which isn't really worth it.

See-also, https://github.com/rust-bitcoin/rust-bitcoin/pull/299

doc/release-notes-16525.md

@@ -0,0 +1,7 @@
+RPC changes
+-----------
+
+Exposed transaction version numbers are now treated as unsigned 32-bit integers
+instead of signed 32-bit integers. This matches their treatment in consensus
+logic. Versions greater than 2 continue to be non-standard (matching previous
+behavior of smaller than 1 or greater than 2 being non-standard).