Darken $gray-600 by 10% for AA contrast (#25123)

Closes #23319. Both `.text-muted` and `.btn-outline-secondary` (and indeed all secondary items) make use of `$gray-600`. New value provides a contrast ratio of 4.69.

scss/_variables.scss

@@ -15,7 +15,7 @@ $gray-200: #e9ecef !default;
 $gray-300: #dee2e6 !default;
 $gray-400: #ced4da !default;
 $gray-500: #adb5bd !default;
-$gray-600: #868e96 !default;
+$gray-600: #6c757d !default;
 $gray-700: #495057 !default;
 $gray-800: #343a40 !default;
 $gray-900: #212529 !default;