Update migration doc with pipelineRunStatus.taskRuns and runs deprecation

[JeromeJu]

Changes

This change is set to WIP and it has been separated from the #5797 on purpose as it as it is subject to change during the discussion and the progress of #5813.

This commit aims to provide users with a guide for dealing with the deprecated status.taskRuns and status.runs during the migration of the pipelineRunStatus.

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Release Notes

NONE

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[JeromeJu]

/kind documentation