bug: Pixhawk 6X on CM4 UART is not detected

### Bug description

`Detector.detect_serial_flight_controllers()` only walks USB `comports()`. On a Holybro CM4 the 6X is on `/dev/serial0`, so the same board that works on Pi 4 USB is invisible.

### Steps to reproduce

1. Connect Pixhawk 6X to a Holybro CM4 UART
2. Boot BlueOS

### Primary pain point(s)

Autopilot manager does not detect the 6X. USB-C to the 6X works as a workaround.

### Additional context

https://discuss.bluerobotics.com/t/pixhawk-6x-not-detected-by-blueos-on-cm4-baseboard/22263

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start at Detector.detect_serial_flight_controllers(), which currently walks USB comports(). Reproduce on a Holybro CM4 with the Pixhawk 6X connected to /dev/serial0, then verify that the autopilot manager detects the board through the UART while retaining USB detection.

Written by the indexing model from the issue text.