refactor: Apply override specifier consistently

Summary:
refactor: Use override for non-final overriders
1551cea2d52cac403ff506a7cc955d8de8fd6f3e
refactor: Remove override for final overriders
d044e0ec7d37bbcdf10bbdb903b9119741c7297d

Two commits are split out from PR16710 to make reviewing easier.

From C++ FAQ:

C.128: Virtual functions should specify exactly one of virtual, override, or final
 Reason Readability. Detection of mistakes. Writing explicit virtual, override, or final is self-documenting and enables the compiler to catch mismatch of types and/or names between base and derived classes. However, writing more than one of these three is both redundant and a potential source of errors.

This is a backport of Core PR18914

Most of the work was done already in D767

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9082