Add index_lfs_objects_on_file synchronous database index

What does this MR do and why?

This MR adds a migration to create the index_lfs_objects_on_file database index synchronously after it has been created on GitLab.com.

Index check

\d index_lfs_objects_on_file

Command output:

Index "public.index_lfs_objects_on_file"  
 Column |          Type          | Key? | Definition   
--------+------------------------+------+------------  
 file   | character varying(255) | yes  | file  
btree, for table "public.lfs_objects" 

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.

Related to #390401 (closed)