Replace $gray-400 value, replace usages with $gray-200

What does this MR do?

• Update $gray-400 hex value from #bababa in variables.scss to match #868686 in gitlab-ui.
• Find/replace all $gray-400s with $gray-200 in GitLab SCSS.

Purpose

The hex values for nearly all color variables were updated in GitLab UI to a) normalize contrast ratios from hue to hue, so for example, $blue-500 and $orange-500 both have ~4.5:1 contrast ratio on $gray-10, and b) ensure the palette is set up for accessibility success. Read gitlab-ui#627 (closed) for more.

Out of scope

This MR will not cover:

• $gl-gray-400, which will be addressed in #231490 (closed).
• Any dark UI gray variables.

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Closes #224119 (closed)