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`.

i18next-parser.config.ts

@@ -21,7 +21,7 @@ export default {
       },
     ],
   },
-  locales: ["en-GB"],
+  locales: ["en"],
   output: "locales/$LOCALE/$NAMESPACE.json",
   input: ["src/**/*.{ts,tsx}"],
   sort: true,