[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

services/terraform/self-host/aws_db.tf

@@ -57,6 +57,7 @@ resource "aws_db_instance" "mariadb" {
   storage_encrypted       = true
   publicly_accessible     = true
   port                    = local.mariadb_port
+  apply_immediately       = true
   skip_final_snapshot     = true
 }