Replace $gray-800 value, replace usages with $gray-700

What does this MR do?

• Update $gray-800 hex value #4f4f4f in variables.scss to match #404040 in GitLab UI.
• Find/replace all $gray-800 with $gray-700.

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-800, 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 #224123 (closed)