[Onboarding] - Update ChecklistItem

[Aaryan1203]

Desired Changes

We need a new endpoint to be able to update a checklist item
It should be under the route /onboarding/checklist/item/:checklistItemId/update

Payload:
Name string
checklistId
Description? String
parentChecklistItemId? String
Subtasks[]

Acceptance criteria

• New endpoint to update a checklistItem
• only admins can update it
• throws if it has been deleted or not found
• add unit tests

Screenshots (as needed)

No response