GH-41787: Update fmpp-maven-plugin output directory (#41788)

### Rationale for this change

Per convention fmpp-maven-plugin should not directly generate files under target/generated-sources but into a subdirectory.

### What changes are included in this PR?

Changing config to output files under `target/generated-sources/fmpp`

### Are these changes tested?

CI

### Are there any user-facing changes?

No
* GitHub Issue: #41787

Authored-by: Laurent Goujon <laurent@apache.org>
Signed-off-by: David Li <li.davidm96@gmail.com>

java/vector/pom.xml

@@ -199,7 +199,7 @@
             <phase>generate-sources</phase>
             <configuration>
               <config>src/main/codegen/config.fmpp</config>
-              <output>${project.build.directory}/generated-sources</output>
+              <output>${project.build.directory}/generated-sources/fmpp</output>
               <templates>${project.build.directory}/codegen/templates</templates>
             </configuration>
           </execution>