deps: Bump json_annotation lower bound to latest, 4.8.1

Otherwise we get the following warning when running build_runner
(wrapped here for readability):

    [WARNING] json_serializable on lib/api/route/users.dart:
    The version constraint "^4.8.0" on json_annotation
    allows versions before 4.8.1 which is not allowed.

I believe that warning was introduced with the json_serializable
upgrade from 6.6.2 to 6.7.0 in f28dd0b / zulip#137.

Author: gnprice

pubspec.yaml

@@ -41,7 +41,7 @@ dependencies:
   # Use with the CupertinoIcons class for iOS style icons.
   cupertino_icons: ^1.0.2
 
-  json_annotation: ^4.8.0
+  json_annotation: ^4.8.1
   http: ^0.13.5
   html: ^0.15.1
   intl: ^0.18.0