Update text and highlight color of OuiCollapsableNav

[joshuarrrr]

No description provided.

[KrooshalUX]

@joshuarrrr this might not be necessary depending on how #904 turns out.

[joshuarrrr]

One of the weird implementation quirks of OuiCollapsibleNav and OuiCollapsibleNavGroup is that there's no attempt to guarantee contrast ratio of text. The prop description for background explicitly notes that only the title text color is adjusted to the background:

oui/src/components/collapsible_nav/collapsible_nav_group/collapsible_nav_group.tsx

Lines 77 to 81 in 1ff6493

	/**
	* Adds a background color to the entire group,
	* applying the correct text color to the `title` only
	*/
	background?: Background;

Instead, the color prop of OuiText is generally used as-is, and most of the example snippets in the OUI doc site extensively use subdued.

So we can either make sure that <OuiText color="subdued"> is generally smarter and "background-aware", or else make sure it has high enough contrast against any of the OuiCollapsibleNavGroup backgrounds. The latter approach may be the way we go with #938