Support WebTransport on Firefox by updating xwt-web

[qoh]

With an aeronet_webtransport client on Firefox, I get this error on connect (via lightyear_webtransport):

TypeError: ReadableStream.getReader: Trying to read with incompatible controller

Which is this issue in the xwt-web dependency: xwt_web_sys fails to create datagram reader on Firefox (#156).
Which was fixed in: Add BYOB / default mode negotiation for readable streams (#239).

The Firefox client works with this change to aeronet:

diff --git i/Cargo.toml w/Cargo.toml
index 1108d07..6d5090c 100644
--- i/Cargo.toml
+++ w/Cargo.toml
@@ -71,7 +71,7 @@ web-sys = { version = "0.3.70" }
 wtransport = { version = "0.7.1" }
 x509-cert = { version = "0.2.5" }
 xwt-core = { version = "0.9.0" }
-xwt-web = { version = "0.20.0" }
+xwt-web = { version = "0.21.2" }
 xwt-wtransport = { version = "0.19.0" }
 
 [workspace.lints.clippy]

You might also want to update other xwt dependencies at the same time, such as xwt-wtransport, but it doesn't seem necessary to fix this.