Add new gray's $gray-25 & $gray-75 to GitLab UI

Why

This is a follow-up to gitlab!87817 (merged) & gitlab!89533 (merged).

In the Design System we currently have $gray-10, $gray-50 & $gray-100 defined. But we currently lack any nuances between these grays. As we overall tend to a softer UI there are places were those could be beneficial – like the contextual navigation.

FYI: In the product we have a $gray-normal which is actally using the same HEX value as the proposed $gray-25.

Proposal

Let's add the following gray's to GitLab UI:

$gray25	$gray75
#f5f5f5	#e9e9e9