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

[alexbegoon]

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

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Checklist

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

[CLAassistant]

All committers have signed the CLA.