Add note re security risk testing older versions

This change adds a note to the section regarding setting up support engineer test instances. It asks the reader to consider any security risk their older gitlab testing versions might introduce, and strongly suggests adding IP filtering to control access if the instance is internet facing.

This is in response to this notification received regarding EXIF exploitation for one of my testing instances, and I thought it would be helpful to have this reminder right before the section where new engineers learn about setting up GitLab test instances.