fix: Remove ember-cli-string-helpers

- This pulls @ember/string 3.1.1 into dependent project and that causes a lot of issues
- Since we have `capitalize` in @ember/string, we can just call that in our internal helper
- Also bumped internal version of @ember/string to v4.x

Fixes #1666

Author: MichalBryxi

package.json

@@ -109,7 +109,7 @@
     "@ember-data/serializer": "^5.3.9",
     "@ember-data/store": "^5.3.9",
     "@ember/optional-features": "^2.2.0",
-    "@ember/string": "^3.1.1",
+    "@ember/string": "^4.0.0",
     "@ember/test-helpers": "^3.3.1",
     "@embroider/test-setup": "^3.0.3",
     "@fullhuman/postcss-purgecss": "^4.1.3",