Add index_packages_package_files_on_file_name sync database index

What does this MR do and why?

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

Index check

\d index_packages_package_files_on_file_name

Command output:

Index "public.index_packages_package_files_on_file_name"  
  Column   |  Type   | Key? | Definition   
-----------+---------+------+------------  
 file_name | integer | yes  | file_name  
gin, for table "public.packages_package_files"  

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 #390335 (closed)