[php] Update Flight to v3 (#9470)

* Update Flight to v3

* Fix plaintext

frameworks/PHP/flight/composer.json

@@ -1,5 +1,5 @@
 {
     "require": {
-        "mikecao/flight": "^2.0"
+        "mikecao/flight": "^3.0"
     }
-}
+}

frameworks/PHP/flight/index.php

@@ -13,9 +13,8 @@ Flight::route('/json', function() {
 // Plaintext test
 Flight::route('/plaintext', function() {
 	Flight::response()
-		->header('Content-Type', 'text/plain')
-		->write('Hello, World!')
-		->send();
+	->header('Content-Type', 'text/plain');
+	echo 'Hello, World!';
 });
 
 // DB test