Update Javadoc stylesheet for JDK6/7

[marcphilipp]

If we decide to move on to JDK 6 or 7 for JUnit 5.0 we need to update the CSS. Maybe use the one from #887.

[junit-builds]

JUnit 4 is now in maintenance mode.

At this point, only critical bugs and security issues will be fixed.

The team has therefore decided to close this issue.