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

[reta]

Is your feature request related to a problem? Please describe

Set notifications 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]

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