p2p: refactor: tidy up PeerManagerImpl::Misbehaving(...)
Summary:
- introduce constant variables score_before and score_after in order to improve readability
- deduplicate calls to LogPrint(), eliminates else-branch
- update BAN to DISCOURAGE in logs (missed in D6506)
This is a backport of core#22495
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13342