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MINOR: [Docs][Java] Add Apache Staging url to docs (#43324)

### Rationale for this change

* The JAR staging location is not documented, let's fix that.

### What changes are included in this PR?

* Add Arrow Java Apache Staging URL to docs.
* Remove lingering JDK 8 references.

### Are these changes tested?

Yes - screenshot shared below

### Are there any user-facing changes?

Yes, improved documentation on the Arrow website.

Authored-by: Dane Pitkin <dpitkin@apache.org>
Signed-off-by: Dane Pitkin <dpitkin@apache.org>