Add banner to clarify the status of this project

Author: snicoll

README.md

@@ -1,3 +1,7 @@
+> ⚠️ **Project Status: Maintenance Only**  
+> This project has been superseded by **Spring Framework 7** and is no longer accepting enhancements.  
+> Please migrate to Spring Framework's [resilience features](https://spring.io/projects/spring-framework).
+
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