chore: drop php 7.1 support (#226)

Author: simPod

.github/workflows/tests.yml

@@ -15,7 +15,6 @@ jobs:
         operating-system:
           - 'ubuntu-latest'
         php-version:
-          - '7.1'
           - '7.2'
           - '7.3'
           - '7.4'

composer.json

@@ -15,7 +15,7 @@
         }
     ],
     "require": {
-        "php": ">=7.1",
+        "php": ">=7.2",
         "psr/http-message": "^1.0",
         "php-http/message-factory": "^1.0",
         "psr/http-factory": "^1.0"