bump dependency to latest major version of the API

[nelsonwittwer]

What this PR does

Bumps the dependency on the shopify_api gem to the latest major version that was recently released. For those looking to install the latest version of the version of the ruby API there was a bundler dependency error as documented in shopify_app #1284

This also follows the direction of the API gem and requires Ruby 3.0 or greater as 2.7 is no longer supported.

Reviewer's guide to testing

To verify this worked as expected I used one of my test apps and installed the latest version of the API and pointed to this local branch:

gem "shopify_api", "~> 14.0"
gem "shopify_app", path: "../../shopify_app"

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

[paulomarg]

Just the one nit, but makes sense to me.