[terraform] add backups to self-host aws rds

Summary:
A previous commit for this had to be reverted. AWS randomly chooses maintainence window times and my selected maintainence window and backup window overlapping, causing the terraform apply to fail.

This diff ensures they are specified differently.

Test Plan: terraform apply. Ensured backup modifications were completed

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13428