Vue 2.7 no longer requires @vue/composition-api

[CaoMeiYouRen]

Please refer to
vuejs/vue@v2.7.0-beta.8...v2.7.0
https://github.com/vuejs/vue/blob/main/CHANGELOG.md

So please modify the part about Vue version judgment in "scripts/postinstall.js"

[JonathanLee-LX]

when i upgrade to vue2.7, some functions type do not work. eg Ref