Change requirement level of network.transport in HTTP semconv

[trask]

Tracking issue for @lmolkova's #373 (comment):

One thing we can consider is to require network.transport presence for HTTP/3 if it's known for sure

I.e. change "If not default (tcp for HTTP/1.1 and HTTP/2, udp for HTTP/3)."

to

"If not default (tcp for HTTP/1.1 and HTTP/2). MUST be set for HTTP/3 if it is known"

This way, a lack of protocol would mean a lack of information (and not default value). We should still be able to change the condition in the future if we'd have reliable ways to determine the protocol.

currently:

semantic-conventions/model/trace/http.yaml

Lines 22 to 24 in b127e12

	- ref: network.transport
	requirement_level:
	conditionally_required: If not default (`tcp` for `HTTP/1.1` and `HTTP/2`, `udp` for `HTTP/3`).

[trask]

follow-up from @lmolkova:

so HTTP/3 is always over QUIC which is always over UPD. QUIC is also a transport protocol and has TCP/2 nickname.

so it seems to be precise, we'd rather set network.transport to QUIC rather than to UDP.
it seems we never really need transport for HTTP as long as we set protocol version