tighten validation logic

[marten-seemann]

We're currently somewhat lenient when validating a WebTransport request, in order to improve compatibility with older WebTransport implementations.

At some point, we should tighten the validation logic to what the (then) RFC version says. In particular:

• only accept webtransport-h3 as upgrade token, see use webtransport-h3 as the :protocol upgrade token #280
• remove the old ENABLE_WEBTRANSPORT setting, see use SETTINGS_WT_ENABLED introduced in draft-15 #254