Migrate i18next-parser config

* Replace useKeysAsDefaultValue with a defaultValue function.
  * Documentation: https://github.com/i18next/i18next-parser/blob/master/docs/migration.md#migrating-from-6x-to-7x
* Remove nsSeparator because it was never a valid config field.

i18next-parser.config.js

@@ -12,8 +12,7 @@ module.exports = {
   input: ['src/**/*.{js,jsx,ts,tsx}', 'pages/**/*.{jsx,tsx}'],
   verbose: true,
   sort: true,
-  nsSeparator: false,
   namespaceSeparator: false,
   keySeparator: false,
-  useKeysAsDefaultValue: true,
+  defaultValue: (locale, namespace, key) => key,
 };