Replace Hamcrest example with AssertJ in User Guide #5050
Description
In the Third-party Assertion Libraries section of the User Guide, we currently have an example using Hamcrest. However, Hamcrest is no longer as popular as it once was.
Since that example was introduced, AssertJ has gained significant adoption within the Java community. In fact, we use AssertJ in JUnit's test suite.
In light of that, we should replace the Hamcrest example with an AssertJ example.