Bumped up Dart min version to 3.0.0 for json_serializable, because json_serializable requires Dart 3.5 since 6.9.0

Author: al-kimmel-serj

modules/openapi-generator/src/main/resources/dart/libraries/dio/pubspec.mustache

@@ -10,7 +10,7 @@ publish_to: {{.}}
 {{/pubPublishTo}}
 
 environment:
-  sdk: '>={{#useJsonSerializable}}2.17.0{{/useJsonSerializable}}{{^useJsonSerializable}}2.15.0{{/useJsonSerializable}} <4.0.0'
+  sdk: '>={{#useJsonSerializable}}3.0.0{{/useJsonSerializable}}{{^useJsonSerializable}}2.15.0{{/useJsonSerializable}} <4.0.0'
 
 dependencies:
   dio: '^5.2.0'

samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-json_serializable/pubspec.yaml

@@ -4,7 +4,7 @@ description: OpenAPI API client
 homepage: homepage
 
 environment:
-  sdk: '>=2.17.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   dio: '^5.2.0'