ci: fix dependency version on PHP7

Author: ph-fritsche

.github/workflows/ci.yml

@@ -11,6 +11,9 @@ jobs:
     strategy:
       matrix:
         php: ['7.4', '8.0']
+        include:
+          - php: '7.4'
+            dependency-versions: 'highest'
 
     steps:
     - uses: actions/checkout@v2
@@ -19,6 +22,8 @@ jobs:
       with:
         php-version: ${{ matrix.php }}
     - uses: ramsey/composer-install@v1
+      with:
+        dependency-versions: ${{ matrix.dependency-versions }}
 
     - run: composer lint
     - run: composer test