Docs needed: [Enhancement] Propagate flat_json config ALTER to BE via versioned task

[github-actions]

Documentation needed for merged PR

PR #74747 was merged and declared a behavior change, but documentation was not added. Please review and update the docs.

PR: [Enhancement] Propagate flat_json config ALTER to BE via versioned task
Author: @moweonlee
Behavior change types declared:

• Policy changes: use new policy to replace old one, functionality automatically enabled
• Miscellaneous: upgrade & downgrade compatibility, etc.

Action required

• Review the PR diff and update the relevant documentation
• Close this issue when documentation is published

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