Dump transaction version as an unsigned integer in RPC/TxToUniv

Summary:

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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10013