Add sync index security_scans_on_pipeline_id_and_scan_type

What does this MR do and why?

This merge request adds a migration to create security_scans_on_pipeline_id_and_scan_type 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 !116010 (merged), was created on GitLab.com.

Resolves #402978 (closed) and #402743 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.