Replace $gray-300 value, replace usages with $gray-200

What does this MR do?

• Updates the $gray-300 hex value from #cccccc to #999999.
• Replaces current usage of $gray-300 with $gray-200.

This MR is part of &2964

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 #224118 (closed)