documentation fix

[prepstarr]

Overview

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it seems to work fine with Java 18, it's time that a program as essential as Junit 5 is up to date and there's no reason not to have it work with Java 18. I corrected the build settings and now I am updating the documentation. I hereby agree to the terms of the JUnit Contributor License Agreement.

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Definition of Done

• [ ] There are no TODOs left in the code
• [ ] Method preconditions are checked and documented in the method's Javadoc
• [ ] Coding conventions (e.g. for logging) have been followed
• [ ] Change is covered by automated tests including corner cases, errors, and exception handling
• [ ] Public API has Javadoc and @API annotations
• [ ] Change is documented in the User Guide and Release Notes
• [ ] All continuous integration builds pass