Replace $gray-600 value, replace usages with $gray-400

What does this MR do?

• Update $gray-600 hex value #919191 in variables.scss to match #5e5e5e in gitlab-ui.
• Find/replace all $gray-600 with $gray-400.

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-600, which will be addressed in #231490 (closed).
• Dark UI.

Does this MR meet the acceptance criteria?

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 #224121 (closed)