Improve formatting of README maintenance note (google#2618)

Uses GitHub-specific Markdown syntax for displaying a "Note" section. See
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

README.md

@@ -5,7 +5,8 @@ Gson can work with arbitrary Java objects including pre-existing objects that yo
 
 There are a few open-source projects that can convert Java objects to JSON. However, most of them require that you place Java annotations in your classes; something that you can not do if you do not have access to the source-code. Most also do not fully support the use of Java Generics. Gson considers both of these as very important design goals.
 
-:information_source: Gson is currently in maintenance mode; existing bugs will be fixed, but large new features will likely not be added. If you want to add a new feature, please first search for existing GitHub issues, or create a new one to discuss the feature and get feedback.
+> [!NOTE]\
+> Gson is currently in maintenance mode; existing bugs will be fixed, but large new features will likely not be added. If you want to add a new feature, please first search for existing GitHub issues, or create a new one to discuss the feature and get feedback.
 
 ### Goals
   * Provide simple `toJson()` and `fromJson()` methods to convert Java objects to JSON and vice-versa