Deprecate nav-divider() mixin

scss/_dropdown.scss

@@ -114,7 +114,7 @@
 
 // Dividers (basically an `<hr>`) within the dropdown
 .dropdown-divider {
-  @include nav-divider($dropdown-divider-bg, $dropdown-divider-margin-y);
+  @include nav-divider($dropdown-divider-bg, $dropdown-divider-margin-y, true);
 }
 
 // Links, buttons, and more within the dropdown menu

scss/mixins/_nav-divider.scss

@@ -2,9 +2,10 @@
 //
 // Dividers (basically an hr) within dropdowns and nav lists
 
-@mixin nav-divider($color: $nav-divider-color, $margin-y: $nav-divider-margin-y) {
+@mixin nav-divider($color: $nav-divider-color, $margin-y: $nav-divider-margin-y, $ignore-warning: false) {
   height: 0;
   margin: $margin-y 0;
   overflow: hidden;
   border-top: 1px solid $color;
+  @include deprecate("The `nav-divider()` mixin", "v4.4.0", "v5", $ignore-warning);
 }