chore: set phpunit-version for the prefer-lowest job

Author: alexislefebvre

.github/workflows/tests.yml

@@ -19,6 +19,7 @@ jobs:
         include:
           - php-version: 7.4
             symfony-version: "^5.4"
+            phpunit-version: '^9.6'
             composer-flags: "--prefer-lowest"
           - php-version: 8.1
             symfony-version: "^5.4"