[FLINK-28861][table][docs] Update "upgrading" documentation to includ…

[dannycranmer]

…e notice regarding the Table API operator issue

What is the purpose of the change

Update documentation to add a notice regarding the Table API operator issue in FLINK-28861

Brief change log

• Updated "upgrading" documentation

Verifying this change

• Visualised locally in IDE

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:

• f3cdb7c Azure: SUCCESS

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