Add index vulnerability_finding_signatures_on_signature_sha concurrently

What does this MR do and why?

This merge request adds a migration to create vulnerability_finding_signatures_on_signature_sha index synchronously.

This is the final step for creating a new index asynchronously as per the guidelines.

This is to ensure the index is created on instances other than GitLab.com.

NOTE: to be only merged after verifying the index, introduced in !115887 (closed), was created on GitLab.com

Resolves #402402 (closed) and #397004 (closed).

As it turns out, the index can be created concurrently in a post-deployment migration, and there's no need to do this asynchronously.

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