Update $gray-500 hex value and replace usages with $gray-300

What does this MR do?

• Update $gray-500 hex value from #a7a7a7 in variables.scss to match hex #666 gitlab-ui.
• Find/replace all instances of $gray-500 with $gray-300.

Out of scope

• Dark theme.
• $gl-gray-500, which is currently separate and will be updated in #231490 (closed).

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 #224120 (closed)