Move checkstyle to profile activated on Java 21+

Checkstyle 13.0.0 requires Java 21+. Move the plugin into a profile
that only activates on Java 21+, so `mvn test` works on Java 17
without loading incompatible classes.

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

Author: horgh

pom.xml

@@ -72,33 +72,6 @@
                   </execution>
                 </executions>
             </plugin>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-checkstyle-plugin</artifactId>
-                <version>3.6.0</version>
-                <configuration>
-                    <consoleOutput>true</consoleOutput>
-                    <configLocation>checkstyle.xml</configLocation>
-                    <suppressionsLocation>checkstyle-suppressions.xml</suppressionsLocation>
-                    <violationSeverity>warning</violationSeverity>
-                </configuration>
-                <dependencies>
-                    <dependency>
-                        <groupId>com.puppycrawl.tools</groupId>
-                        <artifactId>checkstyle</artifactId>
-                        <version>13.0.0</version>
-                    </dependency>
-                </dependencies>
-                <executions>
-                    <execution>
-                        <id>test</id>
-                        <phase>test</phase>
-                        <goals>
-                            <goal>check</goal>
-                        </goals>
-                    </execution>
-                </executions>
-            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-gpg-plugin</artifactId>
@@ -281,5 +254,43 @@
                 </plugins>
             </build>
         </profile>
+        <!-- Checkstyle 13+ requires Java 21+. -->
+        <profile>
+            <id>checkstyle-jdk21</id>
+            <activation>
+                <jdk>[21,)</jdk>
+            </activation>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-checkstyle-plugin</artifactId>
+                        <version>3.6.0</version>
+                        <configuration>
+                            <consoleOutput>true</consoleOutput>
+                            <configLocation>checkstyle.xml</configLocation>
+                            <suppressionsLocation>checkstyle-suppressions.xml</suppressionsLocation>
+                            <violationSeverity>warning</violationSeverity>
+                        </configuration>
+                        <dependencies>
+                            <dependency>
+                                <groupId>com.puppycrawl.tools</groupId>
+                                <artifactId>checkstyle</artifactId>
+                                <version>13.0.0</version>
+                            </dependency>
+                        </dependencies>
+                        <executions>
+                            <execution>
+                                <id>checkstyle</id>
+                                <phase>test</phase>
+                                <goals>
+                                    <goal>check</goal>
+                                </goals>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
     </profiles>
 </project>