Bump flutter_markdown version to latest (#2339)

fixes: https://github.com/flutter/samples/issues/2332

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gemini_tasks/pubspec.yaml

@@ -8,7 +8,7 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
-  flutter_markdown: ^0.7.1
+  flutter_markdown: ^0.7.3
   google_generative_ai: ^0.4.0
   url_launcher: ^6.2.6