docs: update important note with GitHub Flavored Markdown

Author: lauthieb

README.md

@@ -1,4 +1,5 @@
-> **❗ Important note: This current version of Vitamin will no longer evolve and only accept bug fixes from now on. [More details here](https://github.com/Decathlon/vitamin-design/blob/main/IMPORTANT_NOTE.md).**
+> [!IMPORTANT]
+> This current version of Vitamin will no longer evolve and only accept bug fixes from now on. [More details here](https://github.com/Decathlon/vitamin-design/blob/main/IMPORTANT_NOTE.md).
 
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