Use VersionManager#checkpoint instead of VersionManager#checkin

Currently, org.onehippo.repository.documentworkflow.task.VersionVariantTask#doExecute leaves the preview node checked in. We have all over the place invocations of org.hippoecm.repository.util.JcrUtils#ensureIsCheckedOut

Much easier to use VersionManager#checkpoint which does a checkout after checkin...which we always need in practice

Note that all existing code for org.hippoecm.repository.util.JcrUtils#ensureIsCheckedOut is still need for backwards compatibility because preview nodes can already be checked in