Bump ejs and @vue/cli-service in /demo-site

Removes [ejs](https://github.com/mde/ejs). It's no longer used after updating ancestor dependency [@vue/cli-service](https://github.com/vuejs/vue-cli/tree/HEAD/packages/@vue/cli-service). These dependencies need to be updated together.


Removes `ejs`

Updates `@vue/cli-service` from 4.4.1 to 5.0.8
- [Release notes](https://github.com/vuejs/vue-cli/releases)
- [Changelog](https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-cli/commits/v5.0.8/packages/@vue/cli-service)

---
updated-dependencies:
- dependency-name: ejs
  dependency-type: indirect
- dependency-name: "@vue/cli-service"
  dependency-type: direct:development
...

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Author: dependabot[bot]