update webrtc to 0.20

[getong]

Description

AI Assistance Disclosure

Tools used (required — write none if no AI was used): e.g. none / Claude Code / Copilot / Cursor / ChatGPT

Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates