Upgrade to JUnit 6

[sbrannen]

Overview

Since JUnit 6.0 M1 has been released, we should start running our test suite using JUnit 6 for the following reasons.

1. We want to ensure that our own test suite works with JUnit 6 in Gradle and within IDEs.
2. We want to ensure that the SpringExtension and related JUnit support in spring-test works properly with JUnit 6.

Related Issues

• ConditionEvaluationResult APIs should accept @Nullable reasons junit-team/junit-framework#4698
• Add computeIfAbsent variants for non-nullable types to stores junit-team/junit-framework#4760

[sbrannen]

Current work on this effort can be viewed in the following feature branch.

main...sbrannen:spring-framework:junit6

On a positive note, the build in that branch is green! ✅

[sbrannen]

We are now successfully building against JUnit 6.0 RC1: sbrannen@8d11bce