Make the `version` column consistently used in cluster applications
Per our changes in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20010 we introduced another bug in https://gitlab.com/gitlab-org/gitlab-ee/issues/6795#note_86065584 because the existing data was being left around as 2.0.0
which was not a compatible helm chart and this version was meant to refer to the prometheus version anyway. The version column probably should have been nullified.
We probably want to go back and clean up the DB data somehow (nullify or migrate to correct values) then make use of the version
column consistently across all models so people are less confused about this column in future. Or if we have a better way to simplify this then we should do that.
Fixing this should also fix https://gitlab.com/gitlab-org/gitlab-ee/issues/6795 as we need to ensure version is updated correctly on re-install