use maven enforcer plugin for Java version

Added Maven Enforcer Plugin to enforce Java version.

Author: baltzell

pom.xml

@@ -263,6 +263,28 @@
 
     <plugins> <!-- plugins inherited by all child POMs -->
 
+      <!-- Java version enforcer -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <version>3.6.2</version>
+        <executions>
+          <execution>
+            <id>enforce-java</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <requireJavaVersion>
+                  <version>[21,)</version>
+                </requireJavaVersion>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+
       <!-- static analysis -->
       <plugin>
         <groupId>com.github.spotbugs</groupId>