fix webuntis server URL and certificate

lib/main.dart

@@ -46,6 +46,7 @@ Future<void> main() async {
 
     PlatformAssetBundle().load('assets/ca/lets-encrypt-r3.pem').then(addCertificateAsTrusted),
     PlatformAssetBundle().load('assets/ca/lets-encrypt-r10.pem').then(addCertificateAsTrusted),
+    PlatformAssetBundle().load('assets/ca/lets-encrypt-r13.pem').then(addCertificateAsTrusted),
 
     Future(() async {
       await HydratedStorage.build(