Merge branch 'correct-gray-600' into 'master'

Update $theme-gray-600 and $gl-text-color-tertiary values according to Design System

Closes #47645

See merge request gitlab-org/gitlab-ce!19631

app/assets/stylesheets/framework/variables.scss

@@ -99,7 +99,7 @@ $theme-gray-200: #dfdfdf;
 $theme-gray-300: #cccccc;
 $theme-gray-400: #bababa;
 $theme-gray-500: #a7a7a7;
-$theme-gray-600: #949494;
+$theme-gray-600: #919191;
 $theme-gray-700: #707070;
 $theme-gray-800: #4f4f4f;
 $theme-gray-900: #2e2e2e;
@@ -191,7 +191,7 @@ $gl-font-weight-normal: 400;
 $gl-font-weight-bold: 600;
 $gl-text-color: #2e2e2e;
 $gl-text-color-secondary: #707070;
-$gl-text-color-tertiary: #949494;
+$gl-text-color-tertiary: #919191;
 $gl-text-color-quaternary: #d6d6d6;
 $gl-text-color-inverted: rgba(255, 255, 255, 1);
 $gl-text-color-secondary-inverted: rgba(255, 255, 255, 0.85);