chore: Upgrade to JUnit 6

[mshabarov]

No description provided.

[Artur-]

How can the junit 5 version be 6? What happens with all existing projects that use 5?

[mshabarov]

How can the junit 5 version be 6? What happens with all existing projects that use 5?

This was just a naive try to check if TestBench sources still compile and can work with JUnit 6.

Locally I had no big problems to build and validate with JUnit 6, so by running mvn clean install -pl vaadin-testbench-integration-tests-junit5,vaadin-testbench-integration-tests I could see that only these things were failing:

1. ElementScreenCompareIT for JUnit 5 which is unrelated I think.
2. PerformanceIT also unrelated as this might be my local run specific.

Otherwise the project has built and end-to-end tests passed.

I wonder if we can even avoid a new module specifically for JUnit 6 and just say the JUnit 5 based projects can just set JUnit 6 and continue using TestBench.