Ensure that warnings as errors are enabled for CI checks on main

Following the merge of https://github.com/opensearch-project/OpenSearch/pull/9120, publication of maven snapshots started failing due to a compilation warning. On the backport of that change to 2.x, the @SuppressWarnings("unchecked") annotation was added to the method with the compilation warning to ensure that the backport PR passed CI checks.

It looks like there may be a difference between how CI checks are performed on main vs 2.x since it was required to add the annotation in 2.x, but not on main. I'm filing an issue to investigate why there is a difference in the checks between the 2 branches and ensure that these checks are running on the main branch as well as 2.x.

Issue discussed in PR to resolve the maven snapshot publication here: https://github.com/opensearch-project/OpenSearch/pull/9149