fix: increase WebRTC max datachannel message size

[achingbrain]

The 16KB limit was fixed in browsers quite a while back so we should be able to increase the max datachannel message size to 256KB at least.

Fixes #2612

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

Seems like there's an issue with length prefix encoding larger chunks

[maschad]

Seems like there's an issue with length prefix encoding larger chunks

[sukunrt]

Large messages break compatibility with go-libp2p and also possibly with rust-libp2p(https://github.com/libp2p/rust-libp2p/blob/master/misc/webrtc-utils/src/stream.rs#L47). go-libp2p will not read messages larger than 16KB.

This also requires a change in specs. https://github.com/libp2p/specs/blob/1fec30e9fbab36ec687251ea528bf5847723dcff/webrtc/README.md?plain=1#L91