[FLINK-29740][Scala] Mark all Scala APIs as @Deprecated

[MartijnVisser]

What is the purpose of the change

• Mark all Scala APIs as deprecated as the outcome of FLIP-265

Brief change log

• Mark all Scala APIs as @Deprecated
• Remove Scala quickstarts, walkthroughs and examples
• Added deprecation hint to DataStream and Table API overview/API page plus Scala Extension Page

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): yes
• 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:

• d27a5c3 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[twalthr]

I think this PR goes a bit too far. The purpose of a deprecation is to inform users about an outdated class or method. We don't need to deprecate every little corner of an API we if want to drop the entire API anyway. A blog post, an updated roadmap radar, and moving the API web documentation into a dedicated "Legacy APIs" section should be sufficient. Although we deprecate an API, the API should still be usable in an IDE for some time, which is not the case if all API parts are striked through. If at all, we should only deprecate the entry point of the API.

[MartijnVisser]

If at all, we should only deprecate the entry point of the API.

Following our offline sync on this, I've gone with marking the package (which is what you see if you visit https://nightlies.apache.org/flink/flink-docs-master/api/scala/org/apache/flink/api/scala/index.html) and the ExecutionEnvironment https://nightlies.apache.org/flink/flink-docs-master/api/scala/org/apache/flink/api/scala/ExecutionEnvironment.html as @Deprecated

The one thing I'm still doubting a bit is about the Scala documentation examples: should we add a hint warning to all examples (feels like overkill), a generic hint for each page that contains a Scala example or should we remove the Scala examples?

[syhily]

Does this mean that flink won't support Scala by default?

[MartijnVisser]

Does this mean that flink won't support Scala by default?

See https://cwiki.apache.org/confluence/display/FLINK/FLIP-265+Deprecate+and+remove+Scala+API+support -> There won't be dedicated Scala APIs in Flink anymore, but you can still use Scala to write your Flink applications. That's explained in https://flink.apache.org/2022/02/22/scala-free.html

[zentol]

The one thing I'm still doubting a bit is about the Scala documentation examples: should we add a hint warning to all examples (feels like overkill), a generic hint for each page that contains a Scala example or should we remove the Scala examples?

I'd remove the Scala examples and add a warning all pages that are specifically about Scala (like https://nightlies.apache.org/flink/flink-docs-release-1.16/docs/dev/datastream/scala_api_extensions/).

[zentol]

The scala quickstart/walkthrough are still referenced in the documentation.

[MartijnVisser]

@flinkbot run azure

[zentol]

https://nightlies.apache.org/flink/flink-docs-release-1.16/docs/dev/configuration pages could also use some deprecation notices.