[FLINK-29668][Gelly] Remove Gelly

[MartijnVisser]

What is the purpose of the change

• Remove Gelly

Brief change log

• Adds deprecation warning for the DataSet API to all DataSet API documentation, not just the overview page
• Removes all Gelly code
• Removes Gelly documentation
• Adds hint to check out Flink ML Iterations as a replacement for DataSet Iterations support in the DataSet deprecation warning

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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:

• c75cfd1 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[zentol]

There's a gelly reference in the javadocs of org.apache.flink.api.java.utils.DataSetUtils#checksumHashCode.

[tisonkun]

Time past quickly :)

[MartijnVisser]

@flinkbot run azure

[MartijnVisser]

@flinkbot run azure