Bump chartkick from 4.1.3 to 4.2.1
Bump chartkick from 4.1.3 to 4.2.1

Bumps [chartkick](https://github.com/ankane/chartkick) from 4.1.3 to 4.2.1.
- [Release notes](https://github.com/ankane/chartkick/releases)
- [Changelog](https://github.com/ankane/chartkick/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ankane/chartkick/compare/v4.1.3...v4.2.1)

---
updated-dependencies:
- dependency-name: chartkick
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
alphagov/release,alphagov/release,alphagov/release