Fix Simplenote iOS metadata automation not detecting en-US

During the 4.55 finalization, I got this error in ASC


Turns out the `en-US` "What's new" metadata had not been uploaded...

That is confirmed by looking at [the CI logs](https://buildkite.com/automattic/simplenote-ios/builds/1239#0192eebf-00c3-4a8b-af83-3a9df5284183)

```
[14:20:02]: Detected languages: ["ar-SA", "de-DE", "default", "es-ES", "fr-FR", "he", "id", "it", "ja", "ko", "nl-NL", "pt-BR", "ru", "sv", "tr", "zh-Hans", "zh-Hant"]
```

I worked around this manually in the context of 4.55 for time constraints by adding the release notes in ASC myself, but this ought to be fixed before handing over to developers.

---

Two ideas:

- Update Fastlane to latest version—seldom hurts
- Move `copyright.txt` from `metadata/` to `metadata/defaults/` (see the CI logs). Maybe having the file there somehow confuses the use of `metadata/default/` for `en-US`?
- Add _something_ to `metadata/en-US/` (currently empyt) so it's picked up