feat: Add Java 21 support to multilspy

- Update Eclipse JDTLS to vscode-java v1.42.0 with Java 21.0.7 JRE
- Update Kotlin language server dependencies to use Java 21
- Update test Maven project to use Java 21 compiler settings
- Update runtime configuration from JavaSE-17 to JavaSE-21
- Update dependency versions: lombok 1.18.36, launcher 1.7.0

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/resources/repos/java/test_repo/pom.xml

@@ -8,7 +8,22 @@
     <packaging>jar</packaging>
     <name>Java Test Repo</name>
     <properties>
-        <maven.compiler.source>17</maven.compiler.source>
-        <maven.compiler.target>17</maven.compiler.target>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
+        <maven.compiler.plugin.version>3.13.0</maven.compiler.plugin.version>
     </properties>
+    
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>${maven.compiler.plugin.version}</version>
+                <configuration>
+                    <source>21</source>
+                    <target>21</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
 </project>