chore: lint on JDK 21+ only (#641)

Prepare for Checkstyle to require Java 21 or newer.

Author: rhwood

pom.xml

@@ -119,9 +119,9 @@ limitations under the License.
             </activation>
         </profile>
         <profile>
-            <id>Java 17+</id>
+            <id>Java 21+</id>
             <activation>
-                <jdk>[17,)</jdk>
+                <jdk>[21,)</jdk>
             </activation>
             <build>
                 <plugins>