[backport#19512] p2p: banscore updates to gui, tests, release notes

Summary:
fa108d6a757838225179a8df942cfb6d99c98c90 test: update tests for peer discouragement (Jon Atack)
1a9f462caa63fa16d7b4415312d2032a42b3fe0b gui, doc: rm Ban Score in GUI Peers window/release notes updates (Jon Atack)

Pull request description:

This is the third `-banscore` PR in the mini-series described in #19464. See that PR for the intention and reasoning.

- no longer display "Ban Score" in the GUI peers window and add a release note, plus release note fixups per https://github.com/bitcoin/bitcoin/pull/19464#pullrequestreview-447452052
- update tests (`src/test/denialofservice_tests.cpp` and `test/functional/p2p_leak.py`) from banning to discouragement and per https://github.com/bitcoin/bitcoin/pull/19464#issuecomment-658052518

Backport of Core PR19512

Depends on D8799

Test Plan:

ninja all check check-functional

run bitcoin-qt on testnet mode, open Peers window, click a peer and see
that Banscore is no longer reported

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Subscribers: Fabien, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8800