Document GitLab Jenkins Plug-in Compatibility

ZenDesk issue: https://gitlab.zendesk.com/agent/tickets/26263

JenkinsServiceintegrates with the Jenkins GitLab plugin, but we don't make it explicit what version is required to make this work.

It appears that GitLab 8.8.x needs at least v1.1.32 for the commit status API to work (diff here). We should:

1. Document this in the 8.7->8.8 upgrade documentation, release notes, etc.
2. Document this in the Jenkins CI settings configuration

We should also consider adding an automated check in "Test" settings button to verify the version of the plugin. We could work the authors to see if that would be possible.

/cc: @balameb, @ayufan, @dblessing