Remove mentions of ES7 (we're already on ES9 for new features)

[msegado]

ES2016 (ES7) was published last year and the feature set for ES2017 (ES8) was just finalized, so this would already be a proposed ES9 feature. Probably better to just say "ECMAScript" or "ES.Next" so you don't have to change it every year until it's accepted 😉