Update dependency dart to v3

Author: renovate[bot]

example/pubspec.yaml

@@ -6,7 +6,7 @@ description: Demonstrates how to use the disks plugin.
 publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 
 environment:
-  sdk: ">=2.17.6 <3.0.0"
+  sdk: "<4.0.0"
 
 # Dependencies specify other packages that your package needs in order to work.
 # To automatically upgrade your package dependencies to the latest versions