Reduce the chances of testing unrelated batched background migrations

Currently, we select batched background migrations for testing that were created in the last 2 days. This results in testing batched background migrations that are unrelated to the changes in the MR being tested.

Shortening the window of time will reduce the number of unrelated batched background migrations we test, and will reduce the number of MRs that receive a comment from the database-testing pipeline containing unrelated samples.