Rename en-GB language to en (#2866)

Fixes https://github.com/element-hq/element-call/issues/2865

If we want to have GB and US specific terms later on we can then add then in as `en-GB` and `en-US`.

src/@types/i18next.d.ts

@@ -7,7 +7,7 @@ Please see LICENSE in the repository root for full details.
 
 import "i18next";
 // import all namespaces (for the default language, only)
-import app from "../../locales/en-GB/app.json";
+import app from "../../locales/en/app.json";
 
 declare module "i18next" {
   interface CustomTypeOptions {