Rename CheckIfBanned() to MaybeDiscourageAndDisconnect()

Summary:
This completes backport of core PR19219.
This renaming part was previously skipped from D7146 due to missing
backports:
https://reviews.bitcoinabc.org/D7146?id=22789#inline-42357

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8791