Ensure Safari 14.0 compatibility

[stefnnn]

The spree_backend does not work on Safari 14.0.1 (2020) with the new @hotwire/stimulus controllers. The static targets = syntax throws an error. There is an alternative way according to this issue https://github.com/hotwired/stimulus/issues/366#issuecomment-761303692

Tested this locally on both Safari 14.0.1 and Safari 14.1.2 and latest Chrome and it works.

Would be great if you could merge this swiftly and update the npm @spree/dashboard package, as it's quite cumbersome to integrate a forked version.