Remove ember-cli-string-helpers addon

[barryofguilder]

When I upgrade my addon to use @ember/string v4.0, my addon build will throw an error that says:

"Module not found: Error: ember-cli-string-helpers is trying to import the app's @ember/string package, but it seems to be missing"

I noticed that the ember-cli-string-helpers addon was archived in October 2024. I think it would be a good idea to remove this package and replace it with something that is maintained. For now, the way to fix this issue is by locking the version of @ember/string to 3.1.1.