bump php to 8.4 so that Symfony 8 will work

Author: tacman

.github/workflows/ci-linux.yaml

@@ -42,7 +42,7 @@ jobs:
             fail-fast: false
             matrix:
                 php-version:
-                    - '8.3'
+                    - '8.4'
                 symfony-version:
                     - '6.4.*'
                     - '7.3.*'