Fix menu item colors

[jasmussen]

Fixes #15387

This PR fixes an inconsistency in menu item colors. There were some redundant CSS that had not been cleaned up properly with the move to the color mixins.

Additionally it replaces the use of opacity for the lighter text in the menus, and uses the color variables instead, as those colors do not need to work on differently colored backgrounds, just white.

Screenshots:

[gziolo]

There is a related issue with menu hover state missing which could be tackled next: #12262 :)

[m-e-h]

It can be a crapshoot sometimes when, in a mixin, you define a color without also defining the background-color.

@mixin menu-style__neutral() {
	border: none;
	box-shadow: none;
	color: $dark-gray-600;
}

A more predictable and cleaner approach might be to define the color where we're defining the background for both of these components, on .components-popover__content. https://github.com/WordPress/gutenberg/blob/master/packages/components/src/popover/style.scss#L139

Maybe color: inherit in the neutral mixin if we need something there.

[jasmussen]

Good call, thank you. The inherit didn't work out as intended, but it definitely was a good point about not including the color, I actually noticed a few regressions it caused 😱 — so I've restored, but fixed, the colors.

[jorgefilipecosta]

I verified with this changes the shortcuts don't appear with different colors anymore, and they don't use opacity 👍