Merge pull request #38 from contentstack/pubspec-update

version upgrades done

Author: harshithad0703

pubspec.yaml

@@ -6,7 +6,7 @@ documentation: https://www.contentstack.com/docs/developers/apis/content-deliver
 
 environment:
   # sdk: ">=2.17.6 <3.0.0"
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.12.0 <4.0.0"
 
 dependencies:
   dartdoc: ^8.1.0 # dartdoc to generate HTML documentation for your Dart package
@@ -19,9 +19,9 @@ dependencies:
 
 dev_dependencies:
   flutter_lints: ^5.0.0
-  build_runner: ^2.0.4
-  build_test: ^2.1.0
+  build_runner: ^2.4.13
+  build_test: ^2.2.2
   build_web_compilers: ^4.0.11
-  json_serializable: ^6.1.5
+  json_serializable: ^6.9.0
   test: ^1.20.1