Fix no rounded corners if there is only one card in a card-group (#24080)

scss/_card.scss

@@ -207,7 +207,7 @@
           }
         }
 
-        &:not(:first-child):not(:last-child) {
+        &:not(:first-child):not(:last-child):not(:only-child) {
           border-radius: 0;
 
           .card-img-top,
@@ -215,6 +215,10 @@
             border-radius: 0;
           }
         }
+
+        &:only-child {
+          @include border-radius($card-border-radius);
+        }
       }
     }
   }