use php8.4 in phpstan CI job

Author: shu-yusa

.github/workflows/ci.yml

@@ -52,7 +52,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: 8.1
+          php-version: 8.4
           tools: composer
 
       - name: Get Composer cache directory