[junit] provide a pure JUnit5 classpath container

Currently JDT provides a JUNIT5 classpath container that uses the [junit-vintage](https://junit.org/junit5/docs/current/user-guide/#migrating-from-junit4) engine allowing to run Junit5 in parallel to JUnit 3/4:


for project that want to go pure JUnit5 it would be good to strip away the vintage stuff and only have the new JUnit5 items so for example the don't get suggestions from JUnit3/4 API anymore.

To make this non intrusive I would suggest to allow adding an attribute to the container like this:

```

```

If no attribute is given it should assume `true` so existing setups are not affected.