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
Additionally treat Tx.nVersion as unsigned in joinpsbts
This gets its own release note callout, though doesn't appear to
violate the BIP as the BIP appears to be underspecified. We
probably want to update BIP 174 to mention how version numbers are
combined.
This is a backport of core#16525