[Improvement][API] Remove obsolete task update with upstream endpoint

[ruanwenjun]

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Description

The single task definition feature was removed in #16397, but its task update-with-upstream endpoint and dedicated implementation remain. The current UI no longer calls this endpoint. Remove the obsolete API and its exclusive code and tests, and document the incompatible API change.

Are you willing to submit a PR?

• Yes, I am willing to submit a PR.

Code of Conduct

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