DEV: Use Service::Base for suspend and silence actions (#28459)

This commit moves the business logic in the `Admin::UsersController#suspend` and `Admin::UsersController#silence` actions to dedicated service classes. There's no functional changes in this commit.

Internal topic: t/130014.

spec/requests/api/schemas/json/user_silence_request.json

@@ -16,5 +16,9 @@
       "type": "string",
       "example": "delete"
     }
-  }
+  },
+  "required": [
+    "silenced_till",
+    "reason"
+  ]
 }