require php-http/message-factory directly (#261)

This library still uses the deprecated php-http message factories. They have been deprecated since a while in favor of PSR-17 message factories.

php-http/discovery no longer automatically installs php-http/message-factory.

composer.json

@@ -45,6 +45,7 @@
         "php": "^7.2|^8",
         "php-http/client-implementation": "^1",
         "php-http/message": "^1.5",
+        "php-http/message-factory": "^1.1",
         "php-http/discovery": "^1.14",
         "symfony/http-foundation": "^2.1|^3|^4|^5|^6",
         "moneyphp/money": "^3.1|^4.0.3"