The existing message only mentioned one type of before/after callback,
but the config was named generally. That mismatch is confusing and users
wouldn't necessarily know what the total effect of the config would be.
So instead of handwriting the deprecation warning in the specific instances,
consolidate it in one place and give the appropriate context. That context
is the above, but also that users shouldn't update their app config,
they should uncomment the line in the new defaults file, which now also
has more context.
I'm not totally convinced that we can't move this to when
`after_enqueue`/`after_perform` is called in the job class. Doesn't
seem worth it to blare this after every job enqueue/perform, when we
the score at boot time.
cc @Edouard-chin
d258bfe8ac