[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