chore: run PHPStan only in PHP 8.1

Since the version is specified in PHPStan config, there is no need to run in PHP 8.0.

Author: kenjis

.github/workflows/test-phpstan.yml

@@ -34,7 +34,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: ['8.0', '8.1']
+        php-versions: ['8.1']
     steps:
       - name: Checkout
         uses: actions/checkout@v3