chore: update compiler target from JDK 17 to JDK 21

JDK 17 is no longer supported by recent Jenkins versions.
The enforcer plugin already required Java 21+ at runtime,
so this aligns the compiler target accordingly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gounthar

pom.xml

@@ -26,7 +26,7 @@
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.14.1</version>
           <configuration>
-            <release>17</release>
+            <release>21</release>
           </configuration>
         </plugin>
       </plugins>