Remove diff-format and add PHP 8.1

Author: Brandin

.github/workflows/test.yaml

@@ -15,7 +15,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['7.4', '8.0']
+        php: ['7.4', '8.0', '8.1']
 
     steps:
       - uses: actions/checkout@v2

Makefile

@@ -3,7 +3,7 @@
 test: cs-check test-phpunit test-phpspec
 
 cs-check:
-	vendor/bin/php-cs-fixer fix --diff --diff-format=udiff --dry-run
+	vendor/bin/php-cs-fixer fix --diff --dry-run
 
 test-phpunit:
 	vendor/bin/phpunit