Updating the documentation of v04 proto file (#21036)

* Updating the documentation of v04 proto file

The comments for v04 reference the location of the request payload proto file, which was recently(ish) moved from `pkg/trace/pb/span.proto` to `pkg/proto/datadog/trace/span.proto` (see: https://github.com/DataDog/datadog-agent/pull/10650/files#diff-235ec354aa017d69a8115cfc766bfb04271bff2a707e5c6a1c99095e43423207).

This pr corrects the documents so that they point to new location of the proto file.

* Updating v07 payload references as well.

pkg/trace/api/version.go

@@ -23,7 +23,7 @@ const (
 	//
 	// Request: Trace chunks.
 	// 	Content-Type: application/msgpack
-	// 	Payload: An array of arrays of Span (pkg/trace/pb/span.proto)
+	// 	Payload: An array of arrays of Span (pkg/proto/datadog/trace/span.proto)
 	//
 	// Response: Service sampling rates.
 	// 	Content-Type: application/json
@@ -113,7 +113,7 @@ const (
 	//
 	// Request: Tracer Payload.
 	// 	Content-Type: application/msgpack
-	// 	Payload: TracerPayload (pkg/trace/pb/tracer_payload.proto)
+	// 	Payload: TracerPayload (pkg/proto/datadog/trace/tracer_payload.proto)
 	//
 	// Response: Service sampling rates (see description in v04).
 	//