PATCH: Fix Deprecation warning for Dynamic variables access and Dynamic return type

* Fixed Deprecated dynamic variable access warning

* Fixed Deprecated return type will change warning

* Added tests for PHP 8.3

* Handled undefined keys for services

* Updated test fixtures

* Updated library version to PATCH 5.5.1

Author: gocardless-ci-robot[bot]

composer.json

@@ -1,7 +1,7 @@
 {
   "name": "gocardless/gocardless-pro",
   "description": "GoCardless Pro PHP Client Library",
-  "version": "5.5.0",
+  "version": "5.5.1",
   "keywords": [
     "gocardless",
     "direct debit",