change X-B3-Traceid to OTel

[dprotaso]

We're using X-B3-Traceid in various places.

Figure out the OTel equivalent here.

[linkvt]

With regards to the question in the issue description, right now we're quite set using W3C Trace Context, see:

serving/vendor/knative.dev/pkg/observability/tracing/provider.go

Lines 48 to 51 in c46db77

	return propagation.NewCompositeTextMapPropagator(
	propagation.TraceContext{},
	propagation.Baggage{},
	)

This also means that there is currently no possibility to trace requests coming through kourier which only sets the B3 headers.
I created the following PR that allows setting W3C Trace Context headers in kourier, its only allowed since kourier 1.36 though: knative-extensions/net-kourier#1394

In case we also want to continue supporting B3 headers we should maybe add the following context propagator: https://pkg.go.dev/go.opentelemetry.io/contrib/propagators/b3
Guess we would need to test if B3 or W3C Trace Context headers have priority.

[dprotaso]

@linkvt I think we should just cut over to OTel and have Kourier emit those traces and not B3