Bump @mui/x-charts from 7.9.0 to 7.21.0 in /spiffworkflow-frontend (#…

…2125)

Bumps [@mui/x-charts](https://github.com/mui/mui-x/tree/HEAD/packages/x-charts) from 7.9.0 to 7.21.0.
- [Release notes](https://github.com/mui/mui-x/releases)
- [Changelog](https://github.com/mui/mui-x/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mui/mui-x/commits/v7.21.0/packages/x-charts)

---
updated-dependencies:
- dependency-name: "@mui/x-charts"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>