WHY are these changes introduced?

Fixes https://github.com/Shopify/shopify-cli-planning/issues/453

The CLI 2.x will be sunset on April 28, 2023, so we want to warn about that on each command.

WHAT is this pull request doing?

• Adds a warning on each command, including extra information for app, theme, and app/extension creation commands
• Adds a post installation message for Homebrew

How to test your changes?

Just run any command

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).