Bump handlebars from 4.0.12 to 4.7.6 in /defer/client

Bumps [handlebars](https://github.com/wycats/handlebars.js) from 4.0.12 to 4.7.6.
- [Release notes](https://github.com/wycats/handlebars.js/releases)
- [Changelog](https://github.com/handlebars-lang/handlebars.js/blob/master/release-notes.md)
- [Commits](handlebars-lang/handlebars.js@v4.0.12...v4.7.6)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]