chore: remove implicit date-fns dependency from demo page

[TrevorBurnham]

Description

I've been working on removing Cloudscape's heavyweight date-fns dependency: cloudscape-design/components#3786 Currently the CI run for that PR fails because this chart-components package has an implicit dependency on date-fns, which it gets from the main components package. The dependency is only used on a demo page, not in the chart components themselves.

This PR updates the demo page to use vanilla JavaScript for those demo page date calculations instead of date-fns.

How has this been tested?

I've run the local server and the dates on the demo page look good.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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