Update JavaVersionChecker to enforce min of JDK21

[andrross]

Manually tested this:

% JAVA_HOME=~/.sdkman/candidates/java/11.0.27-tem ./bin/opensearch
OpenJDK 64-Bit Server VM warning: Shared spaces are not supported in this VM
OpenSearch requires Java 21; your Java version from [/Users/andrross/.sdkman/candidates/java/11.0.27-tem] does not meet this requirement

Check List

• Functionality includes testing.

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[github-actions]

❌ Gradle check result for aa9cdb9: 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 aa9cdb9: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.59%. Comparing base (6afec2a) to head (aa9cdb9).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...tools/java_version_checker/JavaVersionChecker.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18034      +/-   ##
============================================
+ Coverage     72.57%   72.59%   +0.02%     
- Complexity    67160    67179      +19     
============================================
  Files          5478     5478              
  Lines        310130   310130              
  Branches      45087    45087              
============================================
+ Hits         225068   225145      +77     
+ Misses        66661    66604      -57     
+ Partials      18401    18381      -20     

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