Fixes broken custom MAVLink Action support, including pre-defined Camera Zoom and Focus Actions:

• Transitions built-in users to the new autopilotSystemId instead
• Adds a legacy fallback for users of the old ardupilotSystemId data-lake variable that cannot be migrated
  • This is the "least evil" solution we could come up with, because custom Actions and DIY widgets can be exported (so the problem is unavoidable), and we don't want an indefinite migration tool that needs to detect whenever something refers to the old variable name and update it