fix(deletions): Fix pull request unused query to use exists

[wedamija]

We found that for other deletions, using joins wasn't performant. Switching this over to use exists instead.

[seer-by-sentry]

Potential bug: The Exists subqueries use OuterRef("commit_id"), but the outer PullRequest model does not have a commit_id field, which will cause a FieldError.

• Description: The get_unused_filter class method constructs a query where Exists subqueries for ReleaseCommit and ReleaseHeadCommit use OuterRef("commit_id"). This attempts to reference a commit_id field from the outer PullRequest model's context. However, the PullRequest model does not have a commit_id field. When Django's ORM attempts to build the SQL for this query, it will fail to resolve this reference and raise a FieldError. This will cause the PullRequestDeletionTask, which uses this filter, to crash, preventing the cleanup of old pull request data.

• Suggested fix: The OuterRef("commit_id") incorrectly refers to the outer PullRequest model. The subqueries for commit_in_release and commit_in_head should be constructed to reference the commit_id from the intermediate PullRequestCommit model within the commit_exists subquery. This can be achieved by using a nested OuterRef or by restructuring the query to correctly scope the reference.
  _{severity: 0.8, confidence: 0.98}

_{Did we get this right? 👍 / 👎 to inform future reviews.}