Bump PHP to >=8.0 due to dependency constraint

Also, support for PHP 7 has been discontinued since 03 Nov 2022.

Author: bobvandevijver

README.md

@@ -11,7 +11,7 @@ Requirements
 
 - [Sign up](https://www.messagebird.com/en/signup) for a free MessageBird account
 - Create a new access_key in the developers sections
-- MessageBird API client for PHP requires PHP >= 7.3.
+- MessageBird API client for PHP requires PHP >= 8.0.
 
 Installation
 -----

composer.json

@@ -16,7 +16,7 @@
         "source": "https://github.com/messagebird/php-rest-api"
     },
     "require": {
-        "php": ">=7.3",
+        "php": ">=8.0",
         "ext-curl": "*",
         "ext-json": "*",
         "firebase/php-jwt": "^7.0"