Add variable to configure the `$dropdown-border-radius`. (#24376)

scss/_dropdown.scss

@@ -27,7 +27,7 @@
   background-color: $dropdown-bg;
   background-clip: padding-box;
   border: $dropdown-border-width solid $dropdown-border-color;
-  @include border-radius($border-radius);
+  @include border-radius($dropdown-border-radius);
   @include box-shadow($dropdown-box-shadow);
 }
 

scss/_variables.scss

@@ -488,6 +488,7 @@ $dropdown-padding-y:                .5rem !default;
 $dropdown-spacer:                   .125rem !default;
 $dropdown-bg:                       $white !default;
 $dropdown-border-color:             rgba($black,.15) !default;
+$dropdown-border-radius:            $border-radius !default;
 $dropdown-border-width:             $border-width !default;
 $dropdown-divider-bg:               $gray-200 !default;
 $dropdown-box-shadow:               0 .5rem 1rem rgba($black,.175) !default;