[Security] Bump handlebars from 4.1.0 to 4.7.6 in /frontend

Bumps [handlebars](https://github.com/wycats/handlebars.js) from 4.1.0 to 4.7.6. **This update includes a security fix.**
- [Release notes](https://github.com/wycats/handlebars.js/releases)
- [Changelog](https://github.com/wycats/handlebars.js/blob/master/release-notes.md)
- [Commits](handlebars-lang/handlebars.js@v4.1.0...v4.7.6)

Signed-off-by: dependabot-preview[bot] <[email protected]>