[terraform] Fix lambda opensearch domain access

Summary:
Tried to figure out why we had 10 minute wait times for terraform apply and discovered that I misconfigured the opensearch domain access policy.

The opensearch domain access shouldn't be what determines access for the lambda. Instead, we should separately attach the policy directly to the lambda.

Test Plan: Tested on staging. Added a test user after applying terraform changes and verified the username appeared in our staging search index through websocat.

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11869