Add end-of-life to Gazebo Classic branch

Author: Rick-v-E

README.md

@@ -1,5 +1,8 @@
 # Virtual Maize Field
 
+> [!IMPORTANT]
+> Gazebo Classic reached [end-of-life](https://community.gazebosim.org/t/gazebo-classic-end-of-life/2563) in January 2025, so this branch will no longer receive updates.
+> 
 <p float="left" align="middle">
   <img src="misc/FRE-logo.png" width="250">
 </p>