As described in https://www.mediawiki.org/wiki/Compatibility#General_information, MediaWiki currently requires ES2016 (ES7) for its modern layer. Browsers that do not pass the ES7 feature checks do not receive JavaScript features at all, hence there's no point supporting ES5.

Acceptance criteria

• Use wikimedia/client in our eslint config (an alias for the current minimum)
• Update code to use ES7 features