Smoke tests for instances running OpenSearch and postgres

ActiveContext supports connecting gitlab to either Elasticsearch, OpenSearch or postgres (and more could be added in the future). Because gitlab.com only runs with Elasticsearch, we have a blindspot for issues introduced in a release and don't want customers to report those to us. We need to know about issues before they make it to a release.

Some options:

1. Run gitlab instances with opensearch + one for postgres that gets deployed along with gitlab.com and have smoke tests run on a schedule that checks core functionality and sends a slack message if anything fails.
   1. Notify if a deploy failed
   2. Notify if indexing doesn't work
   3. Notify if searching doesn't work
2. Rely on having thorough E2E tests