Multiple records of Search::Zoekt::Task with task_type delete_repo

What does this MR do and why?

The problem was for the delete_task the project was nil. So find_or_initialize_by(project: project, zoekt_index: zoekt_index) was just returning the first found record with the matching zoekt_index. And then create the delete_repo tasks on that Repository. Thats the reason, we saw multiple tasks records for a specific zoekt_repository because find_or_initialize_by was always returning the same record. Now I am passing the project_id instead of project because for the delete_repo tasks project is nil.

Since the project can be nil in the Search::Zoekt::Repository, the unique validation of zoekt_index_id and project_id is actually not correct. allow: nil was needed. But instead, I have replaced the validation from project_id to project_identifier which is more accurate.

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Before	After

How to set up and validate locally

Enable the feature flag: zoekt_create_indexing_tasks.

Feature.enable :zoekt_create_indexing_tasks

I recommend setting for a new namespace. If you already have Search::Zoekt::EnabledNamespace with existing project_id nil, then you can actually reproduce the bug even when you delete the first project.

Verify on master that it is not working.

1. Setup Zoekt for a namespace
2. Create two projects under the selected root namespace in the first step.
3. Verify in the Rails console that the Search::Zoekt::Repository records are created for these two projects.
4. Now delete one of these two projects from the settings or the rails console. If deleting from the settings you have to go to the settings again to delete the project immediately.
5. Verify in the Rails console that the project_id in the Search::Zoekt::Repository for the deleted project is nil. Also, check that the Search::Zoekt::Task record is created with the task_type as delete_repo for the deleted project.
6. Now delete the second project also. Verify in the Rails console that the project_id in the Search::Zoekt::Repository for the deleted project is nil. Check in the Rails console that Search::Zoekt::Task is created but the project_identifier is equal to the first deleted project.

Verify on the branch 450736-backend-for-multiple-matches-in-a-single-file

In the last step the Search::Zoekt::Task is created but the project_identifier is equal to the second deleted project.

Related to #467154 (closed)