WebTransport no longer defines wt.datagrams.writable

[gsnedders]

Test List

See web-platform-tests/wpt#58539:

/webtransport/datagram-bad-chunk.https.any.html
/webtransport/datagram-bad-chunk.https.any.serviceworker.html
/webtransport/datagram-bad-chunk.https.any.sharedworker.html
/webtransport/datagram-bad-chunk.https.any.worker.html
/webtransport/datagrams.https.any.html
/webtransport/datagrams.https.any.serviceworker.html
/webtransport/datagrams.https.any.sharedworker.html
/webtransport/datagrams.https.any.worker.html
/webtransport/stats.https.any.html
/webtransport/stats.https.any.serviceworker.html
/webtransport/stats.https.any.sharedworker.html
/webtransport/stats.https.any.worker.html

And proposing to add:

/webtransport/historical.https.sub.any.html
/webtransport/historical.https.sub.any.serviceworker.html
/webtransport/historical.https.sub.any.sharedworker.html
/webtransport/historical.https.sub.any.worker.html

Rationale

w3c/webtransport@3e37d39 changed the spec in March 2025.

Note this causes significant regressions for both Chrome and Firefox — but continuing to only implement what the spec hasn't said in a year while Safari is about to ship conforming to the spec (per the WebKit IDL) would clearly be a failure of interoperability.