caddyhttp: Make use of http.ResponseController (#5654)

* caddyhttp: Make use of http.ResponseController

Also syncs the reverseproxy implementation with stdlib's which now uses ResponseController as well 2449bbb5e6

* Enable full-duplex for HTTP/1.1

* Appease linter

* Add warning for builds with Go 1.20, so it's less surprising to users

* Improved godoc for EnableFullDuplex, copied text from stdlib

* Only wrap in encode if not already wrapped

caddytest/integration/stream_test.go

@@ -176,9 +176,7 @@ func testH2ToH2CStreamServeH2C(t *testing.T) *http.Server {
 
 		w.Header().Set("Cache-Control", "no-store")
 		w.WriteHeader(200)
-		if f, ok := w.(http.Flusher); ok {
-			f.Flush()
-		}
+		http.NewResponseController(w).Flush()
 
 		buf := make([]byte, 4*1024)