refactor: Rename Chain::Notifications methods to be consistent with other interfaces methods

Summary:
This also simplifies #10102 removing overrides needed to deal with inconsistent
case convention

Backport of Core PR18278 part [3/6] : https://github.com/bitcoin/bitcoin/pull/18278/commits/6ceb21909ce66b7b4762a855889acd46bb6b77f3

Depends on D8020

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8021