GH-41787: Update fmpp-maven-plugin output directory

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

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

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>