Add a data migration to fix some missing timestamps in the members table (again)

Repeats an earlier migration to fix historic bad data in the members table (missing created_at and updated_at fields)

I'm expecting the WHERE clauses to be fast enough, and to return few enough rows, that the migration doesn't need to use batches, but I'm not too familiar with the size of these tables in the wild, so perhaps that's a poor assumption.

8.10 introduced a dependency on the  `members.created_at` field in the project and namespace member view. If bad data is present, viewing the list of members now results in an NoMethodError and a 500 response from GitLab. Although the previous migration should have fixed all bad rows, we have evidence that it didn't in at least one case, despite the migration claiming to have run in the past.

Closes #20568

See merge request !5670
Signed-off-by: Rémy Coutable <remy@rymai.me>