Prevent database deadlocks when deleting projects

What does this MR do and why?

Related to #346172 (closed) and #346172 (comment 851079961)

Manual pipelines can have cancelable jobs that are not aborted before attempting to delete the pipeline which enqueues workers for processing, resulting in database deadlocks.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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