ci: drop unsupported PHP versions until reactphp#175 gets merged

Author: bpolaszek

.github/workflows/ci.yml

@@ -19,11 +19,6 @@ jobs:
           - 7.3
           - 7.2
           - 7.1
-          - 7.0
-          - 5.6
-          - 5.5
-          - 5.4
-          - 5.3
     steps:
       - uses: actions/checkout@v4
       - uses: shivammathur/setup-php@v2