refactor: remove the warning box

README.md

-<div style="border-left: 4px solid #ffc107; background-color: #fff3cd; color: #856404; padding: 10px; margin-top: 10px;">
-<strong>Notice: This Repository Is No Longer Maintained</strong>
+
+**Notice: This Repository Is No Longer Maintained**
 
 We would like to inform all users that this repository is no longer maintained. For the latest versions, please visit our new repository at the following link:
  
 https://git.rwth-aachen.de/mbd/psimpy
-</div>
 
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