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

[cwperks]

Following the merge of #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: #9149

[cwperks]

I've noticed that when I build with JDK19 that it does not output compile warnings, but after switching to JDK17 it does.

[cwperks]

The workflow that started failing with the compiler warning explicitly uses JDK 17: https://github.com/opensearch-project/OpenSearch/blob/main/.github/workflows/publish-maven-snapshots.yml

[cwperks]

If core is not already running CI checks with multiple supported JDK versions then it could be beneficial to do so. I believe this error did not occur on main, but did on 2.x because the checks were run with different JDK versions.

[cwperks]

Run ./gradlew :libs:opensearch-core:compileJava from 70e1b2274bf6c1245063442882e561d4fb37742a using JDK 17 and you receive the warning.

Run it again with JDK 19 and you no longer receive the warning.

[afrodidact]

I want to take this as a good first issue for my osci-2023 contribution.