Add support of ESP32C6 to OTBR example (CON-1513)

[alexbegoon]

trafficstars

Description

Add support of ESP32C6 target to OTBR example

Testing

I use two XIAO ESP32C6 boards connected via UART. I tested them with Home Assistant and Google Home. ESP-IDF v5.3.1

---

Checklist

• [x] 🚨 This PR does not introduce breaking changes.
• [x] All CI checks (GH Actions) pass.
• [x] Documentation is updated as needed.
• [x] Tests are updated or added as necessary.
• [x] Code is well-commented, especially in complex areas.
• [x] Git history is clean — commits are squashed to the minimum necessary.

[CLAassistant]

All committers have signed the CLA.