Updated baseline

1.538 is over 13 months old and tests cannot even be run on Java 8. Updating to the next newer LTS.
> LTS is preferred as baseline

But why? I suggest using weekly release that was used as a baseline for that LTS to make sure we are using API available in 1.554.1 as well as in 1.555. Requiring LTS does not guarantee anything like that, only causes confusion IMO (_why maven plugin does not work with my 1.554.2?_).