Add missing transition to .form-select (#34034)

A selectbox is styled in a very similar way as an input (`.form-control`), including border-color and box-shadow. So it can be assumed it should apply the same CSS transition for these properties when focused, but this was missing.

Co-authored-by: alpadev <2838324+alpadev@users.noreply.github.com>

scss/forms/_form-select.scss

@@ -22,6 +22,7 @@
   border: $form-select-border-width solid $form-select-border-color;
   @include border-radius($form-select-border-radius, 0);
   @include box-shadow($form-select-box-shadow);
+  @include transition($form-select-transition);
   appearance: none;
 
   &:focus {