Bump org.jmonkeyengine:jme3-core from 3.8.1-stable to 3.9.0-stable (#51)

Bumps [org.jmonkeyengine:jme3-core](https://github.com/jMonkeyEngine/jmonkeyengine) from 3.8.1-stable to 3.9.0-stable.
- [Release notes](https://github.com/jMonkeyEngine/jmonkeyengine/releases)
- [Commits](jMonkeyEngine/jmonkeyengine@v3.8.1-stable...v3.9.0-stable)

---
updated-dependencies:
- dependency-name: org.jmonkeyengine:jme3-core
  dependency-version: 3.9.0-stable
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]