arrow

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>