fix: Downgrade version of @ember/string

- I can't make following test work. No matter what I do it blows up:
```
pnpm ember try:one embroider-safe
```

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": "^4.0.0",
+    "@ember/string": "^3.1.1",
     "@ember/test-helpers": "^3.3.1",
     "@embroider/test-setup": "^3.0.3",
     "@fullhuman/postcss-purgecss": "^4.1.3",