Require guzzlehttp/psr7 >= 1.7.0 for Util class

The PodioClient class depends on the `\GuzzleHttp\Psr7\Utils` class,
however this was not included in the existing requirements, since the
`guzzlehttp/guzzle` requirement of `>=6.2.0` only requires
`guzzlehttp/psr7:~1.1`, so this will cause a "Class not found" error
when attempting to load the Util class.

composer.json

@@ -24,6 +24,7 @@
   "require": {
     "php": "^7.3 || ^8.0",
     "guzzlehttp/guzzle": ">=6.2.0",
+    "guzzlehttp/psr7": ">=1.7.0",
     "ext-json": "*"
   },
   "suggest": {