Delete WorkItemUpdateTask mutation
What does this MR do and why?
This mutation was introduced as alpha so it doesn't need to follow the deprecation process. The mutation is no longer needed so we are removing it.
As described in #434863 (comment 1694790954) the frontend usage was removed in 16.7 so it's safe to remove in 16.9
MR acceptance checklist
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Related to #435042 (closed)
Edited by Mario Celi