Change toast border radius to use `$border-radius` (#28621)

Currently, `$toast-border-radius` uses the value of `.25rem`. This is inconsistent with other variables such as `$btn-border-radius` and `$input-border-radius`, which uses the "global" `$border-radius` variable.

scss/_variables.scss

@@ -898,7 +898,7 @@ $toast-color:                       null !default;
 $toast-background-color:            rgba($white, .85) !default;
 $toast-border-width:                1px !default;
 $toast-border-color:                rgba(0, 0, 0, .1) !default;
-$toast-border-radius:               .25rem !default;
+$toast-border-radius:               $border-radius !default;
 $toast-box-shadow:                  0 .25rem .75rem rgba($black, .1) !default;
 
 $toast-header-color:                $gray-600 !default;