Add missing border-radius for btn-group (#35467)

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/_button-group.scss

@@ -34,6 +34,8 @@
 }
 
 .btn-group {
+  @include border-radius($btn-border-radius);
+
   // Prevent double borders when buttons are next to each other
   > .btn:not(:first-child),
   > .btn-group:not(:first-child) {