Prevent upgrading more than one minor version at a time

Created by: slimsag

I have seen two customers in the past day incorrectly upgrade more than two minor versions at one time, leading to migration failures taking down their instance.

This is a failure on our part, because nobody should have to be realizing this on their own (or through our (missing!) documentation #7699 ): If you upgrade Sourcegraph more than two minor versions at once and we require you don't, you should be getting an obvious error explaining you need to do it in increments.