merge_requests_finder.rb

When adding a `Approved By` or `Approver` filter and sorting by
milestone, a 500 error will occur because of a SQL error:

```
PG::GroupingError: ERROR:  column "milestones.due_date" must appear in
the GROUP BY clause or be used in an aggregate function
```

This happens because the approval query uses a `GROUP BY` query to
find merge requests that match all approvers. However, this `GROUP BY`
query breaks the SQL query since it's expected that
`milestones.due_date` and `milestones.id` are present in that clause.

We can solve this problem in a few ways:

1. Add the required grouping columns to this query (this commit).
2. Use a CTE to isolate the query for approvers.
3. Rewrite the approvers query to avoid the GROUP BY altogether.

The second way appears to work, but there's a risk of running into
errors due to nested queries. Plus, it does not appear the CTE
improves performance.

The third way may also be possible, but it would possibly require some
schema changes or other refactoring.

For now, we can solve this problem by manually adding the required
grouping columns when approvers are specified.

Relates to #223062

Changelog: fixed