Validate merge_request_diffs.project_id foreign key

What does this MR do and why?

Validates merge_request_diffs.project_id foreign key, which was validated asynchronously in !140410 (merged).

Async validation has completed:

Table "public.merge_request_diffs"
       Column        |           Type           | Collation | Nullable |                     Default                     | Storage  | Compression | Stats target | Description 
---------------------+--------------------------+-----------+----------+-------------------------------------------------+----------+-------------+--------------+-------------
 id                  | integer                  |           | not null | nextval('merge_request_diffs_id_seq'::regclass) | plain    |             

<snip>

Foreign-key constraints:
    "fk_56ac6fc9c0" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    "fk_8483f3258f" FOREIGN KEY (merge_request_id) REFERENCES merge_requests(id) ON DELETE CASCADE

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Before	After

How to set up and validate locally

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

Related to #429174 (closed)