[terraform] [services] Change opensearch to t3.small

Summary:
Changes instance size from medium to small to save on costs while opensearch instance isn't under heavy use.
This is the smallest available size as t3.micro isn't available in us-east-2.

Test Plan: terraform plan and fmt

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10656

services/terraform/modules/shared/opensearch.tf

@@ -29,7 +29,7 @@ resource "aws_opensearch_domain" "identity-search" {
   engine_version = "OpenSearch_1.0"
 
   cluster_config {
-    instance_type = "t3.medium.search"
+    instance_type = "t3.small.search"
   }
 
   vpc_options {