[terraform] create automated MariaDB backups

Summary: This one line change enables backups on our aws rds mariadb database.

Test Plan:
Will not land until I've confirmed that after wiping database, I'm able to restore our database point in time to a reasonable prior state (AWS says this is typically 5 minutes).

Deleted the database. The time frame given wasn't 5 minutes, it was actually an hour after but I believe this time varies.
Was able to initialize a new db instance prior to deleting the database and confirm data was there

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13170