[gRPC/http2 auto-detect] Support auto-detection (or upgrade) in Activator

Describe the feature

This is part of #4283.
As part of supporting http2 auto-detect, we need to make changes in health probes, queue proxy, and activator. Below are some notes created by @igorbelianski on the work that needs to be done specifically in activator.

Data Path Notes

Activator always listens on both 8012 an 8013 constructs 2 distinct servers:

  [cmd/activator/main.go](https://github.com/knative/serving/tree/master/cmd/activator/main.go)

     servers := map[string]*http.Server{
	"http1":   pkgnet.NewServer(":"+strconv.Itoa(networking.BackendHTTPPort), ah),
	"h2c":     pkgnet.NewServer(":"+strconv.Itoa(networking.BackendHTTP2Port), ah),
          "profile": profiling.NewServer(profilingHandler),
	}

Queue proxy container construction

While construction queue proxy container we make a static choice on the port 8012/8013
pkg/reconciler/revision/resources/queue.go

func makeQueueContainer(rev *v1.Revision, cfg *config.Config) (*corev1.Container, error) {

Start with cmd/activator/main.go to understand the separate HTTP/1 and h2c servers, then inspect pkg/reconciler/revision/resources/queue.go and makeQueueContainer for the static port choice. Trace how these components participate in HTTP/2 auto-detection, and confirm that activator and queue proxy no longer require the static selection described in the issue.