Fix update service_desk_setting with project_key

What does this MR do and why?

See #383728 (closed)

update service_desk_setting which has project_key will raise an error

How to set up and validate locally

1. create service_desk_setting with project_key
2. run service_desck_setting.valid?

or:

1. open a project
2. open settings > general > Service Desk
3. edit it with a random suffix and save
4. click save changes again

MR acceptance checklist

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