[terraform] Enable apply immediately parameter for aws rds changes

Summary:
Before we do an instance size upgrade, we should enable "apply immediately" as a parameter for our aws rds instance so we can ensure downtime occurs on apply and not later

Depends on D13506

Test Plan: after specifying apply immediately, upgrading instance size through terraform.tfvars.json immediately applys as confirmed by aws console

Reviewers: bartek, varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13507