[Crashlytics] bump upload-symbols version to 3.15 (#11136)

Author: themiswang

Crashlytics/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- [fixed] Updated upload-symbols to 3.15. Disable dSYM uploads for Flutter apps building with --obfuscate. Please upload dSYMs through the Firebase console web uploader. To manually upload your dSYMs, you can use the console-based "Drag and Drop" option to upload a zip archive containing your dSYM files (go to Firebase console > [Crashlytics](https://console.firebase.google.com/project/_/crashlytics) > dSYMs tab). (#11136)
+
 # 10.7.0
 - [fixed] Updated upload-symbols to 3.14 with an improvement to upload all dSYM files for Flutter apps