[FLINK-15633][build] Fix building Javadocs on JDK 11

[NicoK]

What is the purpose of the change

Javadocs won't build when building for Java 11, e.g. like this:

./mvnw clean package javadoc:jar -Pjava11 -Pjava11-target -pl flink-dist -am -Drat.skip=true -Dcheckstyle.skip=true -Dspotless.apply.skip -DskipTests

Brief change log

• add missing add-exports for java.rmi/sun.rmi.registry to make it work

Verifying this change

Verified with the build command from above

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 22d8929 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[zentol]

Good stuff; I thought we always had to bump the plugin! Maybe some other change removed the need in the meantime.

[NicoK]

FYI: test failure is due to FLINK-25306