Accept Guzzle 6 as well as Guzzle 7

composer.json

@@ -15,7 +15,7 @@
     "minimum-stability": "dev",
     "require": {
         "php": ">=7.2",
-        "guzzlehttp/guzzle": "^7.2"
+        "guzzlehttp/guzzle": "^6.0|^7.0"
     },
     "support": {
         "source": "https://gitlab.res-telae.cat/restelae/php-gbif"