[BUG] The Integration tests do not clean up after themselves

[kimpepper]

What is the bug?

The integration tests don't clean up the indexes they create. There is a method to do this in \OpenSearch\Tests\Utility::cleanUpCluster() but it is never called.

This was discovered through the phpstan warnings generated in #259 ✨

::cleanUpCluster() also calls ::deleteAllSLMPolicies() which tries to call a non-existent legacy slm endpoint which would throw an error.

How can one reproduce the bug?

What is the expected behavior?

Integration tests clean up after themselves

What is your host/environment?

Main dev branch.

Do you have any screenshots?

Do you have any additional context?