Create satellites during update to 6.0. Fix #4938

Satellites for projects that were located in the root namespace needs to be recreated once they've been migrated to the creator’s namespace during the upgrade process.

This can be achieved by running the sudo -u git -H bundle exec rake gitlab:satellites:create RAILS_ENV=production during the update process.

The command has been added to the 5.4 to 6.0 update doc.