Update to AssertJ 4.0

[marcphilipp]

Overview

Try out AssertJ 4.0 prior to its release.

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I hereby agree to the terms of the JUnit Contributor License Agreement.

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Definition of Done

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

[marcphilipp]

Looks like xmlunit needs to be adjusted due to https://github.com/assertj/assertj/commit/989faf8daf8933a4ecd2fab60f668c360ff7eb91

[scordio]

Looks like xmlunit needs to be adjusted due to https://github.com/assertj/assertj/commit/989faf8daf8933a4ecd2fab60f668c360ff7eb91

Thanks for the heads up! We'll reach out to the project about it.