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GH-39037: [Java] Remove (Contrib/Experimental) mention in Flight SQL (#39040)

### Rationale for this change

Considering that Flight SQL has been present for a while and should be fairly stable, remove the `(Contrib/Experimental)` mention from the pom file which also shows up on Maven Central UI pages

### Are these changes tested?

Local test but there's no code change, only cosmetic

### Are there any user-facing changes?

None

* Closes: #39037

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