Remove ldap_sync columns from namespaces table and move them to dedicated ldap-settings related table

Namespaces table is very wide, and used in many places, it's basically backbone of GitLab. Because it used so many times and holds much information, sometimes it contributes to the slow performance.

Removing ldap_sync-related columns and moving them to dedicated ldap-settings related table would make the table slimmer and probably more performant thanks to that.

Availability & Testing

Ensure the group_ldap_sync_spec and admin_ldap_sync_spec end-to-end tests pass.