docs: remove README link to non-existent upgrade guide

[zachlysobey]

Related Issues

• No release notes or upgrade guide for 1.0 #5043
• Upgrade Guide needs to be updated with 0.x.x -> 1.0.0 #5014
• Upgrade guide is out dated  #3633
• Delete UPGRADE_GUIDE.md #5087

Description

This PR removes a broken UPGRADE_GUIDE.md link from the (1.x) README.

Really, the guide should be added/updated, but I think it may be better to just omit it until the doc is ready.
A broken link is worse than no link.

Alternatives considered

1. Write an upgrade guide: I don't think I have the context
2. Create a stub guide: Personally I think this is less good than just not having a link, but perhaps it'd make the eventual future addition of such a guide easier. If that's the preferred option, I can open a PR to do that instead.