update Dart samples

Author: wing328

samples/openapi3/client/petstore/dart-dio/binary_response/.openapi-generator/VERSION

@@ -1 +1 @@
-7.14.0-SNAPSHOT
+7.15.0-SNAPSHOT

samples/openapi3/client/petstore/dart-dio/binary_response/README.md

@@ -4,7 +4,7 @@ No description provided (generated by Openapi Generator https://github.com/opena
 This Dart package is automatically generated by the [OpenAPI Generator](https://openapi-generator.tech) project:
 
 - API version: 1.0.0
-- Generator version: 7.14.0-SNAPSHOT
+- Generator version: 7.15.0-SNAPSHOT
 - Build package: org.openapitools.codegen.languages.DartDioClientCodegen
 
 ## Requirements