[FLINK-24055][connectors/kafka] Deprecate FlinkKafkaConsumer

[fapaul]

What is the purpose of the change

• Deprecate FlinkKafkaConsumer and remove docs with link to 1.13 docs

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Automated Checks

Last check on commit e96eeeb (Mon Aug 30 14:36:13 UTC 2021)

Warnings:

• Documentation files were touched, but no docs/content.zh/ files: Update Chinese documentation or file Jira ticket.

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• e96eeeb Azure: SUCCESS

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[AHeise]

LGTM.