Bump chart.js from 2.9.3 to 2.9.4 in /web-ui

Bumps [chart.js](https://github.com/chartjs/Chart.js) from 2.9.3 to 2.9.4.
- [Release notes](https://github.com/chartjs/Chart.js/releases)
- [Commits](chartjs/Chart.js@v2.9.3...v2.9.4)

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

Author: dependabot[bot]