lib/upgrade: 0.x to 1.0 is a minor upgrade

This removes the special handling of minor versions as major when the
actual major is zero, and adds the special case that upgrades from 0.x
to 1.x are considered minor. 0.x to 2.x or 1.x to 2.x etc are still
considered major.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4226
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