## Because
- `supported-languages.json` lists `en-US`, but the l10n repo only ships `en`, `en-CA` and `en-GB`. Every page load asked for `.../en-US/main.ftl` and got a 404, so the request could never be a cache hit.
- Negotiation made that unavoidable. `en-US` matched itself, and a plain `en` header expanded to every `en-*` entry in the list, which pulled `en-US` in too.
## This pull request
- Drops `en-US` from `supported-languages.json`. Fluent strips the region and resolves `en-US` to `en`, so we serve the English bundle we actually deploy.
- Adds a case to `supported-languages.spec.ts` that keeps `en-US` out of the list. JSON holds no comments, so the test carries the reason.
- Adds `parseAcceptLanguage` and `determineLocale` cases for `en-US`, and updates the existing expectations in `l10n.utils.spec.ts`.
- Moves the `en-US` fixtures in `AppLocalizationProvider.test.tsx` and `NimbusContext.test.tsx` to `en-CA`, a locale we do deploy.
- Adds a `NimbusContext` case for the region change described below.
I picked removal over an `EN_GB_LOCALES`-style alias. An alias fixes the `en-US` header but not the `en` header, which still expands to `en-US` through the available-range match, so the 404 would stay.
## Issue that this pull request solves
Closes: https://mozilla-hub.atlassian.net/browse/FXA-10363