Make AnalyzePSBT next role calculation simple, correct

Summary:

Set next to"finalizer" when all inputs need finalizing
Note that this assumes that PSBT roles are a complete ordering, which for now and in the future seems to be a correct assumption.

This is a backport of Core PR18224

Test Plan:
ninja && ninja check && test/functional/test_runner.py rpc_psbt

Check that before this patch to psbt.cpp the functional test fails because the "next" role is signer, and after this patch is has the correct "finalizer" value.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8778