Migrate gl-text-gray-400 to design tokens

What does this MR do and why?

Migrate gl-text-gray-400 to design tokens

• Update gl-text-gray-400 to variant="subtle" on <gl-icon>
• Update gl-text-gray-400 to gl-text-disabled
• Update gl-text-gray-400 to gl-text-subtle

References

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• &15844

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Related to #503085 (closed)