The Issues, Merge Requests and Notes test webhooks were susceptible to
timeouts due to the oldest records being selected to be in the test
webhook payload.

Indexes on these tables seem to favour sorting newest to oldest, the
difference between selecting the oldest Issue, Merge Request, or Note vs
the newest for `gitlab-org/gitlab` is ~ 25 seconds (or sometimes more)
on `#database-lab` (which is slower than production, but indicative) vs
~10 milliseconds when selecting the newest records.

Finders are now used for all scoping to make the scoping even safer.

https://gitlab.com/gitlab-org/gitlab/-/issues/290116