Ensure jdk 21 minimum compatibility across all sub-projects and create replacement for java.security.Policy which is marked for removal

[cwperks]

Description

This PR ensure jdk 21 minimum compatibility across all sub-projects (a few projects had it set to Java 8) and create replacement for java.security.Policy which is marked for removal

Related Issues

Related to #18339

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 fa5211c: 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 c0d489c: null

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 9f25041: 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 9f25041: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 3.26087% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.99%. Comparing base (4ab0ae9) to head (9f25041).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/java/org/opensearch/secure_sm/policy/Policy.java	0.00%	85 Missing ⚠️
...va/org/opensearch/gradle/OpenSearchJavaPlugin.java	0.00%	2 Missing ⚠️
...rg/opensearch/gradle/OpenSearchTestBasePlugin.java	0.00%	1 Missing ⚠️
...rg/opensearch/tools/cli/plugin/PluginSecurity.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19631      +/-   ##
============================================
+ Coverage     72.96%   72.99%   +0.02%     
+ Complexity    70655    70570      -85     
============================================
  Files          5725     5726       +1     
  Lines        323697   323764      +67     
  Branches      46875    46882       +7     
============================================
+ Hits         236198   236330     +132     
+ Misses        68436    68357      -79     
- Partials      19063    19077      +14     

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