Set lower bound of enforced Java versions to 21.

Author: leerho

pom.xml

@@ -190,7 +190,7 @@ under the License.
               <configuration>
                 <rules>
                   <requireJavaVersion>
-                    <version>${java.version}</version>
+                    <version>21,</version> <!-- java.version -->
                   </requireJavaVersion>
                   <requireMavenVersion>
                     <version>[${maven.version},4.0.0)</version>