.github/workflows/test.yml: run phpstan on PHP 8.4

Author: zozlak

.github/workflows/test.yml

@@ -34,8 +34,11 @@ jobs:
   phpstan:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
-    - uses: actions/cache@v2
+    - uses: actions/checkout@v4
+    - uses: shivammathur/setup-php@v2
+      with:
+        php-version: 8.4
+    - uses: actions/cache@v4
       with: 
         path: vendor
         key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}