Jenkins builds migrated

[Andreas Beeker <ki...@apache.org>]

Hi devs,

I've migrated the builds to the new Jenkins environment https://ci-builds.apache.org/job/POI/

There are 2 differences to the old environment:

- The githubPullRequest plugin is not available anymore and Infra doesn't want to install a 3rd github plugin.
  I've tried to configure the cloudbees pendant, but I'm not sure if it would actually work and opposed to the githubPullRequest plugin one can't define Whitelist or trigger phrases. Please someone who is relying on this feature have a look on it.

- The matrix build used in 'POI-DSL-Test-Environment' doesn't support elastic axis anymore and the label expression axis behaves differently, therefore one needs to check the available nodes every now and then and select them explicitly.

I'll now remove all but the github build from the old jenkins.
We need to update the references in the docs too.

Andi