[docs] fix Dart Plugin issue tracker URL (#8784)

Updated to point to the Dart Plugins (new) home.

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

README.md

@@ -46,8 +46,8 @@ for Flutter IntelliJ issues.
 
 - for more general Flutter issues, you should prefer to use the Flutter
   [issue tracker](https://github.com/flutter/flutter/issues)
-- for more Dart IntelliJ related issues, you can use JetBrains'
-  [YouTrack tracker](https://youtrack.jetbrains.com/issues?q=Subsystem:%20%7BLang.%20Dart%7D%20%20State:%20Unresolved)
+- for more Dart IntelliJ related issues, please use the Dart Plugin
+  [issue tracker](https://github.com/flutter/dart-intellij-third-party/issues)
 
 ## Known issues