[framework] 补充 JDK 21 的支持标志

Author: CodeCasterX

README.md

@@ -8,6 +8,7 @@ Spring 双模运行，支持插件热插拔与智能聚散部署，无缝统一
 
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://opensource.org/license/MIT)
 [![JDK](https://img.shields.io/badge/JDK-17-green.svg)](https://www.oracle.com/java/technologies/javase/jdk17-archive-downloads.html)
+[![JDK](https://img.shields.io/badge/JDK-21-green.svg)](https://www.oracle.com/java/technologies/javase/jdk21-archive-downloads.html)
 </div>
 
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