document internal postgres version upgrades

[bdols]

I attempted an upgrade of a install that was frozen on chart 1.12.6 (harbor 2.8.6) to 1.16.2 (harbor 2.12.2) and it seems to me that this may not be supported because the database migration failed and the harbor instance's db was empty and initialized on postgres15. The harbor-db container in 2.12.2 doesn't contain postgres13 but does contain postgres 14 and postgres 15. I'm not sure how the database migration is supposed to work, but I don't see where this is documented.

If this is the case, I suggest that the helm upgrade documentation (https://goharbor.io/docs/2.12.0/administration/upgrade/helm-upgrade/) and/or the chart's README can have notes on postgres db requirements and/or intermediate upgrade needs.

Another possible solution could be to add a helm hook that prevents the upgrade if the previous chart version isn't greater than a specific version, but I'm not sure if that is fully implementable with helm.