Update package:frontend_server_client constraint to ^4.0.0 (#4202)

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index eb197db..3fe357c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -11,7 +11,7 @@
   collection: ^1.18.0
   convert: ^3.1.1
   crypto: ^3.0.3
-  frontend_server_client: ^3.2.0
+  frontend_server_client: ^4.0.0
   http: ^1.1.2
   http_multi_server: ^3.2.1
   http_parser: ^4.0.2