Remove unwanted java checks for JDK<21 in build.gradle

[sandeshkr419]

Description

Since JDK 21 is the minimum Java version required to build up OpenSearch, build checks related to older versions do not hold value in main branch.

Related Issues

Resolves nothing, code cleanup.

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 6d3a876: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for e805b6f: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 3c888a1: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.25%. Comparing base (6b35c7c) to head (3c888a1).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19858      +/-   ##
============================================
+ Coverage     73.08%   73.25%   +0.16%     
- Complexity    71063    71168     +105     
============================================
  Files          5754     5754              
  Lines        325262   325262              
  Branches      47033    47033              
============================================
+ Hits         237732   238255     +523     
+ Misses        68380    67870     -510     
+ Partials      19150    19137      -13     

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[cwperks]

Thank you for helping clean up some of these old references to keep the code base up to date :)

I started on some of the work #19631, but also started replacing references to java.security.Policy in the PR as its marked for removal in the JDK and OpenSearchPolicy subclasses it so it needs a replacement at some point.

The changes in this PR LGTM.

[sandeshkr419]

@cwperks I missed looking at your changes earlier, but I guess your changes will be probably smaller now.