Update dependency guzzlehttp/guzzle to ^7.12.3

Author: mend-for-github-com[bot]

composer.json

@@ -31,7 +31,7 @@
   },
   "require": {
     "php": ">=7.2",
-    "guzzlehttp/guzzle": "^7.0",
+    "guzzlehttp/guzzle": "^7.12.3",
     "nexmo/client-core": "^2.0"
   }
 }