[Demo] Upgrade Echarts to v6

[fzaninotto]

Problem

Echarts v5 has some security vulnerabilities. Dependabot raises alerts for them, even though we only use it in the demo.

Solution

Upgrade it to v6 and remove it from ra-core, where it's only used in a story.

How To Test

• launch the demo and check that the chart renders properly on the dashboard
• launch the storybook and check that the chart appears correctly in http://localhost:9010/?path=/story/ra-core-controller-list-withlistcontext--chart

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• [ ] The PR includes unit tests
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date