Fix .btn-group-vertical border radius with dropdown as first element (#40488)

Co-authored-by: Julien Déramond <juderamond@gmail.com>

scss/_button-group.scss

@@ -135,7 +135,12 @@
     @include border-bottom-radius(0);
   }
 
-  > .btn ~ .btn,
+  // The top radius should be 0 if the button is:
+  // - the "third or more" child
+  // - the second child and the previous element isn't `.btn-check` (making it the first child visually)
+  // - part of a btn-group which isn't the first child
+  > .btn:nth-child(n + 3),
+  > :not(.btn-check) + .btn,
   > .btn-group:not(:first-child) > .btn {
     @include border-top-radius(0);
   }