[FEATURE] Support JDK 21 for main branch

[derek-ho]

Is your feature request related to a problem?
Coming form: opensearch-project/security#4457, the security backend plugin's jdk version was recently changed to 21 on main. When not running JDK 21 in the dashboard plugin's CI, we see errors like

Exception in thread "main" java.lang.IllegalStateException: opensearch-security requires Java 21:, your system: 11.0.23+9-LTS

Link:
https://github.com/opensearch-project/security-dashboards-plugin/actions/runs/9549400899/job/26318874294?pr=1941.
What solution would you like?
Change the JDK versions used in dashboard CI to 21 for main branch
What alternatives have you considered?
None
Do you have any additional context?
No

[stephen-crawford]

[Triage] Hi @derek-ho, for filing this issue. This seems like a good request based on the overall campaign. Will mark as triaged.