Support Rails 7

[gonzaloriestra]

WHY are these changes introduced?

Fixes #1813

WHAT is this pull request doing?

• Uses the version 18.1.0 of shopify_app which adds support for Rails 7
• Avoids installing webpacker for Rails > 7

How to test your changes?

1. Ensure you have Rails 7 (rails -v)
2. shopify app create rails --name rails7app
3. cd rails7app && shopify app serve

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.

[amcaplan]

I tested it locally and it seems to work. Thanks for working through this!