ci: Add PHP 8.1 through 8.4

- 8.1 was introduced in 66215a6c80
- 8.2 and 8.3 was introduced in 9bdd3b6b2e
- 8.4 cherry picked from commit 5b9551d1e3

.github/workflows/continuous-integration.yml

@@ -28,6 +28,10 @@ jobs:
           - "7.3"
           - "7.4"
           - "8.0"
+          - "8.1"
+          - "8.2"
+          - "8.3"
+          - "8.4"
 
     steps:
       - name: "Checkout"