3dsets Model7 Buggy servo arm adpter

Remix of this [Model7 Buggy servo arm adapter](https://www.thingiverse.com/thing:4852615) to work better with the servo horns I purchased on Amazon: https://www.amazon.com/dp/B07D56FVK5.

Changes:
- Holes for mounting the servo arm increased to 3mm
- Center hole enlarged to allow mounting the adapter between the horn and the servo to reduce the required clearance inside the the servo housing on the buggy.
- Decrease the size of the holes for mounting the steering rods so that the screws have more to bite into. They seemed a little loose on the original.