Replace $gray-darkest SCSS variable

What does this MR do and why?

• Replace $gray-darkest SCSS variable with $gray-200

Note: $gray-200 is automatically inverted to $gray-700 value for dark mode in design tokens https://gitlab.com/gitlab-org/gitlab-ui/-/blob/1c3d3138d23d91328848b8370e83e811a4818aaa/src/tokens/color.tokens.json#L149-153

Related MRs

• Replace $gray-lightest SCSS variable with $gray-10 !153381 (merged)
• Replace $gray-lighter SCSS variable with $gray-10 !153390 (merged)
• Replace $gray-light SCSS variable with $gray-10 !153380
• Replace $gray-normal SCSS variable with $gray-50 !153384 (merged)
• Replace $gray-dark SCSS variable with $gray-100 !153386 (merged)
• Replace $gray-darker SCSS variable with $gray-50 !153388 (merged)
• Replace $gray-darkest SCSS variable with $gray-200 !153389 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
$gray-darkest	$gray-200
#bfbfc3	#bfbfc3

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #430055