RFR: 8369049: [TestBug] Update JUnit to Version 6.0.0

[Marius Hanl]

JUnit 6.0.0 is out and we can consider updating to it.

Release Notes: https://docs.junit.org/current/release-notes/index.html#release-notes-6.0.0

Notes:
- Java 17 is the baseline (so not a problem for us, since we have a higher baseline)
- Deprecation were removed (not a problem, as we don't rely on any)
- JUnit Vintage Engine is deprecated (not a problem, as we dropped support for it some months ago)

JUnit 6 now uses a single version number for all dependencies, that is platform, jupiter and vintage (which we do not use anymore). That makes updating it easier.

It also uses `JSpecify`, which is an annotation library for `@NonNull` and `@Nullable`. All version numbers for JUnit now use a single variable.

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Commit messages:
 - Update to JUnit Version 6.0.0

Changes: https://git.openjdk.org/jfx/pull/1925/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1925&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369049
  Stats: 59 lines in 3 files changed: 10 ins; 1 del; 48 mod
  Patch: https://git.openjdk.org/jfx/pull/1925.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1925/head:pull/1925

PR: https://git.openjdk.org/jfx/pull/1925