Default CI Java Version to Java 11, run tests on 8, 14 and 17

[dblock]

Coming from opensearch-project/opensearch-plugins#64, default CI Java version to 11 (LTS), and run tests on 8, 14 and 17.

• Change the default Java version for CI in GitHub actions from 14/15 to 11 (LTS).
• Add Java 8, 14 (used before), and 17 (next LTS) to the CI matrix.
• Document Java versions in DEVELOPER_GUIDE.md.

[zhongnansu]

by adding to "CI matrix", does it mean github CI matrix? It confuses me with the 1st bullet point. Are we using a fixed version or a matrix in github action CI workflow for plugins? @dblock

[zhongnansu]

I tried adding java 8 to the matrix in gitub CI by "setup-java/@v2", I saw the following error. It seems from the doc the opensearch core is expecting the compile time java to be at least 11, while for runtime it could be 8, 11, 14. Need some advice from infra on how to add to github action CI. I think it should be similar across plugins

* What went wrong:
A problem occurred evaluating root project 'opensearch-reports-scheduler'.
> Failed to apply plugin 'opensearch.opensearchplugin'.
   > Could not create plugin of type 'PluginBuildPlugin'.
      > Could not generate a decorated class for type PluginBuildPlugin.
         > org/opensearch/gradle/plugin/PluginPropertiesExtension has been compiled by a more recent version of the Java Runtime (class file version 54.0), this version of the Java Runtime only recognizes class file versions up to 52.0

[dblock]

This is expected, we only want to run tests on 8 but compiling on 11 is fine. I am not sure exactly how to do this in CI (yet).

[dblock]

@davidcui1225 You still need a 1.3.0 build on JDK 8, don't see it in the workflows.

[davidcui1225]

@davidcui1225 You still need a 1.3.0 build on JDK 8, don't see it in the workflows.

Are we pushing back the release date? The code freeze is EOD today, we will only have ~2 hours to get these all merged in.