Add index_merge_request_diffs_on_external_diff sync database index

What does this MR do and why?

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

Index check

\d index_merge_request_diffs_on_external_diff

Command output:

Index "public.index_merge_request_diffs_on_external_diff"  
    Column     |       Type        | Key? |  Definition     
---------------+-------------------+------+---------------  
 external_diff | character varying | yes  | external_diff  
btree, for table "public.merge_request_diffs"  

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 #390399 (closed)