Remove project_import_level column

What does this MR do and why?

This change removes the project_import_level column from the namespace_settings table. It was added with !91875 (merged) when we intended to make the permission to import projects configurable in the UI. However, we now limit this feature to Maintainers and above, so the column is no longer needed (see: !117374 (merged))

This has been added as a post-deployment migration as per our DB migration guidelines

DB Migration output

main: == [advisory_lock_connection] object_id: 183240, pg_backend_pid: 54941
main: == 20231220091345 RemoveProjectImportLevelFromNamespaceSettings: migrating ====
main: -- remove_column(:namespace_settings, :project_import_level, :smallint, {:default=>50, :null=>false})
main:    -> 0.0023s
main: == 20231220091345 RemoveProjectImportLevelFromNamespaceSettings: migrated (0.0096s) 

main: == [advisory_lock_connection] object_id: 183240, pg_backend_pid: 54941
ci: == [advisory_lock_connection] object_id: 183480, pg_backend_pid: 54943
ci: == 20231220091345 RemoveProjectImportLevelFromNamespaceSettings: migrating ====
ci: -- remove_column(:namespace_settings, :project_import_level, :smallint, {:default=>50, :null=>false})
ci:    -> 0.0016s
ci: == 20231220091345 RemoveProjectImportLevelFromNamespaceSettings: migrated (0.0169s) 

ci: == [advisory_lock_connection] object_id: 183480, pg_backend_pid: 54943

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. checkout into the branch
2. run bundle exec rake db:migrate

Related to #388906 (closed)