Set the chart min-upgrade version to be GitLab 13
Aligns with the upgrade path described in https://docs.gitlab.com/ee/policy/maintenance.html#version-12-onwards-extra-step-for-major-upgrades
As far as I know, this path isn't needed for 13.0, but for now we should follow the documented process.
The matching omnibus change is here: gitlab-org/omnibus-gitlab!4270 (merged)