Require Maven 3.9

The Njord build extension does not provision on older Maven: CodeQL's
bundled 3.8.9 fails it with a Guice ProvisionException.  The POM still
asked only for 3.0.5, which predates the extension by a decade.

Both current Ubuntu runner images ship 3.9, and CodeQL passes
-Denforcer.skip, so neither is affected.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M5JVWJN7L3NWmcsYvjeCfc

Author: gaul

pom.xml

@@ -296,8 +296,10 @@
             </goals>
             <configuration>
               <rules>
+                <!-- The Njord extension fails to provision on older
+                     Maven; 3.8.9 dies with a Guice ProvisionException. -->
                 <requireMavenVersion>
-                  <version>3.0.5</version>
+                  <version>3.9</version>
                 </requireMavenVersion>
                 <!-- Checkstyle and Error Prone both ship Java 21 class
                      files, so a full build needs a JDK that new.  Compiling