Bump chart.js from 2.6.0 to 2.9.4 in /Dashboard

Bumps [chart.js](https://github.com/chartjs/Chart.js) from 2.6.0 to 2.9.4.
- [Release notes](https://github.com/chartjs/Chart.js/releases)
- [Commits](chartjs/Chart.js@v2.6.0...v2.9.4)

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

Author: dependabot[bot]