Darken table border-color

Fixes #25016 which noticed a .thead-light and border-color of the .table-border are the same color.

scss/_variables.scss

@@ -302,7 +302,7 @@ $table-hover-bg:              rgba($black, .075) !default;
 $table-active-bg:             $table-hover-bg !default;
 
 $table-border-width:          $border-width !default;
-$table-border-color:          $gray-200 !default;
+$table-border-color:          $gray-300 !default;
 
 $table-head-bg:               $gray-200 !default;
 $table-head-color:            $gray-700 !default;