chore: remove 8.4 until 1.8.9 release

Author: imorland

.github/workflows/REUSABLE_backend.yml

@@ -25,7 +25,7 @@ on:
         description: Versions of PHP to test with. Should be array of strings encoded as JSON array
         type: string
         required: false
-        default: '["7.3", "7.4", "8.0", "8.1", "8.2", "8.3", "8.4"]'
+        default: '["7.3", "7.4", "8.0", "8.1", "8.2", "8.3"]'
 
       php_extensions:
         description: PHP extensions to install.