[Release] arrow-java-root-18.3.0-source-release.zip includes JNI binaries

### Describe the bug, including details regarding any error messages, version, and platform.

This is not a release blocker.

https://github.com/apache/arrow-java/releases/download/v18.3.0-rc2/arrow-java-root-18.3.0-source-release.zip

JNI binaries are large. Can we remove them from `source-release.zip`?

```console
$ LANG=C du -hsc arrow-java-root-18.3.0/jni/*
236K arrow-java-root-18.3.0/jni/arrow_cdata_jni/
272M arrow-java-root-18.3.0/jni/arrow_dataset_jni/
156M arrow-java-root-18.3.0/jni/arrow_orc_jni/
444M arrow-java-root-18.3.0/jni/gandiva_jni/
871M total
```

`source-release.zip` also has `testing/`. It's not so large but should we remove it? Or should we keep it?

```console
$ LANG=C du -hsc arrow-java-root-18.3.0/testing
8.8M arrow-java-root-18.3.0/testing
8.8M total
```

Start with the v18.3.0-rc2 source-release.zip and inspect how its jni/ and testing/ directories are assembled. Determine whether JNI binaries and testing/ belong in the source archive, then verify the resulting archive no longer includes the unwanted binaries and remains a valid source release.

Written by the indexing model from the issue text.