make composer validation workflow use --strict (OpenMage#1478)

Author: Flyingmana

.github/workflows/php.yml

@@ -19,7 +19,7 @@ jobs:
       run: composer --version
 
     - name: Validate composer.json and composer.lock
-      run: composer validate
+      run: composer validate --strict
 
     - name: Install dependencies
       run: composer install --prefer-dist --no-progress --no-suggest --ignore-platform-reqs