Migrate to Junit5

[8Keep]

We still use junit 4. Trying to migrate to junit 5.

This is a bulky all in one PR.

[github-actions]

🖼️ Screenshot tests have failed.

The purpose of these tests is to ensure that changes introduced in this PR don't break visual features. They are visual unit tests.

📄 Where to find the report:

• Go to the (failed run) > Summary > Artifacts > screenshot-test-report
• Download the zip and open jme3-screenshot-tests/build/reports/ScreenshotDiffReport.html

⚠️ If you didn't expect to change anything visual:
Fix your changes so the screenshot tests pass.

✅ If you did mean to change things:
Review the replacement images in jme3-screenshot-tests/build/changed-images to make sure they really are improvements and then replace and commit the replacement images at jme3-screenshot-tests/src/test/resources.

✨ If you are creating entirely new tests:
Find the new images in jme3-screenshot-tests/build/changed-images and commit the new images at jme3-screenshot-tests/src/test/resources.

Note; it is very important that the committed reference images are created on the build pipeline, locally created images are not reliable. Similarly tests will fail locally but you can look at the report to check they are "visually similar".

See https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-screenshot-tests/README.md for more information

Contact @richardTingle (aka richtea) for guidance if required

[gemini-code-assist]

Code Review

This pull request migrates the project's test suite from JUnit 4 to JUnit 5 (Jupiter). It updates the Gradle dependencies, including the addition of the JUnit BOM and platform launcher, and refactors numerous test files to use the new JUnit 5 annotations and assertion classes. A critical issue was identified regarding an invalid version for the JUnit BOM in the version catalog.

[8Keep]

@richardTingle It looks like the screenshot changes are all minuscule changes, let me know if you disagree.