[Java] Fix java doc building error

[chaokunyang]

Why are these changes needed?

When build java doc with jdk11, we got the code being documented uses modules error. This PR fix it by using jdk8 and upgrade java doc plugin version.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[raulchen]

have you figured out the correct way to set jdk8?

[chaokunyang]

Configurign jdk8 in linux is easy, using language: java and jdk: openjdk8. But it doesn't work on macos.

[chaokunyang]

Install jdk8 on macosx manually