Fix MergeRequestDiff.verifiables scope

What does this MR do and why?

Backport the MergeRequestDiff.verifiables scope fix from !210607 (diffs) to %18.6. Without it the pluck_verifiable_ids_in_range(range) returns all merge_request_diffs with external storage, not just those in the batch range, resulting in slow queries. For more context, see https://gitlab.com/gitlab-com/request-for-help/-/issues/4062#note_3017821232.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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