[FEATURE] Set alerting plugin 3.0.0 baseline JDK version to JDK-21

[reta]

Is your feature request related to a problem? Please describe

Set alerting plugin 3.0.0 baseline JDK version to JDK-21

Describe the solution you'd like

See please opensearch-project/OpenSearch#10745

Related component

Build

Describe alternatives you've considered

N/A

Additional context

See please opensearch-project/OpenSearch#14011

[dblock]

[Catch All Triage - Attendees 1, 2, 3, 4, 5]