Ref: https://php.watch/versions/8.0/final-private-function#final-private
This applies #712 from upstream libraries.
Unrelated: I've also changed a occurrence of "private final"
by the correct (PSR-12) "final private" in the constructor
(the final&private combination is allowed in constructors)
in lib/classes/event/base.php, because it's the unique case
in core and phpcs was really insisting.