chore(colors): Fix gl-bg-gray-500

What does this MR do?

Changes the value of mixin gl-bg-gray-500 from $gray-600 to $gray-500

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.

Security

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.