deps.ffmpeg: Update libdatachannel to 0.21.0

[Sean-Der]

This updates to libdatachannel 0.21.0

Motivation and Context

• This fixes (30.1.2) WHIP streaming will cause serious picture damage when a tiny number of packets are lost. obs-studio#10564
• Adds support for WHEP (Accepting H264 and Opus)
• Add supports for STUN/TURN (Implement WHIP spec more)
• Bug fixes and crashes (not reported by OBS users)

How Has This Been Tested?

Tested on Linux. I am looking for Mac/Windows testers now

Types of changes

• Bug fix (non-breaking change which fixes an issue) -->
• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[kc5nra]

Tested on windows and works -

[Sean-Der]

Tested on NixOS and works here!