Remove CourseOverview.get_from_id[s]_if_exists (#22918)

The functions `CourseOverview.get_from_id_if_exists`
and `CourseOverview.get_from_ids if exists` are bug-
prone, in that they do not go to the modulestore
when a CourseOverview is out-of-date or uncached.
The docstrings spell this out, but callers seem
not to understand the implications.

So, in this commit, we:
* Remove both functions
* Add `CourseOverview.get_from_ids` for almost-as-fast
  yet safer bulk-fetching of overviews
* Update call sites to use `course_exists` or `get_from_ids`
  in place of the old functions.
* Add tests for @verify_course_exists.
This PR should specifically fix an issue where
the program_enrollments API was breaking when the CourseOverview
version was bumped.

CR-1708