修复 Paper API 1.21.1 兼容性：Java 17 升级至 Java 21

Paper 1.21.1 API (class version 65) 要求 Java 21+ 运行时，
pom.xml compiler source/target 及 java.version 统一升级至 21

CLAUDE.md

@@ -23,7 +23,7 @@
 
 ## 技术规范
 
-- Java 17，Spring Boot 3.2.0，Maven 3.9.6 (bundled)
+- Java 21，Spring Boot 3.2.0，Maven 3.9.6 (bundled)
 - 编译：`./apache-maven-3.9.6/bin/mvn compile`
 - 测试：`./apache-maven-3.9.6/bin/mvn test`
 - 打包：`./apache-maven-3.9.6/bin/mvn package -DskipTests`

pom.xml

@@ -20,9 +20,9 @@
     </parent>
 
     <properties>
-        <java.version>17</java.version>
-        <maven.compiler.source>17</maven.compiler.source>
-        <maven.compiler.target>17</maven.compiler.target>
+        <java.version>21</java.version>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <smile.version>2.6.0</smile.version>
         <commons-math3.version>3.6.1</commons-math3.version>
@@ -136,8 +136,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.11.0</version>
                 <configuration>
-                    <source>17</source>
-                    <target>17</target>
+                    <source>21</source>
+                    <target>21</target>
                 </configuration>
             </plugin>
             <plugin>