update sdk constraints

Author: lohanidamodar

templates/dart/pubspec.yaml.twig

@@ -6,7 +6,7 @@ repository: https://github.com/{{sdk.gitUserName}}/{{sdk.gitRepoName}}
 issue_tracker: https://github.com/appwrite/sdk-generator/issues
 documentation: {{ spec.contactURL }}
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 dependencies:
   http: '>=0.13.6 <2.0.0'