Default CI Java Version to Java 11, run tests on 8, 14 (and 17) #288
Description
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.