[Packwerk 3.0] Update UPGRADING.md with directions to upgrade to 3.0

[alexevanczuk]

Let's make sure folks have an easy path to migration. We might consider baking some of the upgrade capabilities into packwerk itself, such as automated commands to migrate deprecated_references.yml to package_todo.yml.

Here are a list of things to include:

• migrating to package_todo.yml
• requiring the extracted privacy checker from packwerk-extensions.
• if you were using methods like Packwerk::Package#enforce_privacy?, you'll want to use API available in packwerk-extensions
• updating user docs to reference bin/packwerk update-todo rather than bin/packwerk update-deprecations
• Removal of bin/packwerk detect-stale-violations, which is now replaced with bin/packwerk check

[gmcgibbon]

I'll add some quick docs for the behaviour of deleting deprecated_references.yml automatically and reference your gem for those who still want privacy checks. Thank you for reminding me with the milestone. ❤️