Fix deprecation warnings for Rails 7.1+

[kirillplatonov]

What this PR does

Rails 7.1+ no longer provides public method ActiveSupport::Deprecation.warn.

➜  color-swatches git:(main) ✗ bin/rails test
/Users/kirill/.local/share/mise/installs/ruby/3.4.5/lib/ruby/gems/3.4.0/gems/shopify_app-23.0.0/lib/shopify_app/logger.rb:11:in 
'ShopifyApp::Logger.deprecated': private method 'warn' called for class ActiveSupport::Deprecation (NoMethodError)

        ActiveSupport::Deprecation.warn("[#{version}] #{context(:warn)} #{message}")
                                  ^^^^^

This PR switches to the use of instance of ActiveSupport::Deprecation. to resolve the issue.

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.

[lizkenyon]

Let's mark this as private for consistency with the other helper methods

[kirillplatonov]

Sure, updated

[lizkenyon]

Thanks for your contributions here!

I will release these changes on Monday.