Rename JUnit 5 to JUnit Jupiter in the docs

[nlisker]

ArquillianExtension docs say "Implements several JUnit 5 extension API interfaces to adapt JUnit 5 tests for Arquillian."

This should be updated to remove the confusion with JUnit 6.

There might be more classes that reference it.

[rhusar]

+1 @nlisker and this is already done in PR #809