[EuiCollapsibleNavGroup] background="dark" should be re-written with Emotion colorMode="dark"

[cee-chen]

EuiCollapsibleNavGroup's background="dark" & background="light" props were written 3+ years ago and only really handles setting a background color and not actually setting the correct expected colors on all nested children.

Now that we have Emotion and <EuiThemeProvider colorMode="dark">, we should be able to far more holistically handle this behavior.

This PR may have some overlapping requirements with #6663 and #6759, depending on whether the new Kibana nav design requires a permanently 'dark' UI.

Acceptance criteria

• EuiCollapsibleNavGroup styles should be rewritten in Emotion. EuiCollapsibleNav as a whole may need to be converted to Emotion as well, depending on how nested its styles are
• background prop logic should be written using EuiThemeProvider, and prop docs should be updated