Add not null constraint for releases.project_id
What does this MR do and why?
Adds a NOT NULL constraint for releases.project_id. This is required because project_id is the sharding key for this table.
This is a non-optional association in the model, and there are no existing records on GitLab.com where this column is null (source).
So that we can be confident adding the constraint won't fail, we first delete any existing records that have no project_id. This is expected to be a no-op on all instances, as these records have always been created in the context of a project and don't make sense to exist without one (their existence wouldn't be visible to users).
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