UPGRADING.md

UPGRADING.md was not updated for the 1.8.0 release. I actually didn't even realize we were putting notes about specific upgrade paths in a per-release way inside our release.

This is prone to being overlooked and forgotten. We should have generic version-agnostic information in UPGRADING.md, and put more specific per-release notes in an UPGRADING section on the release notes for specific upgrade issues. At the very least, the version of this document for the current release does not need to contain upgrade instructions for upgrading to previous releases.

The more information we have in our documentation which is written to correspond to the current/next release, the more (undocumented) manual steps are involved for a release manager, and the more likely these steps are going to get overlooked and forgotten.