Remove button border radius when used in a button group (#7834)

Author: cado1982

src/Umbraco.Web.UI.Client/src/less/components/buttons/umb-button-group.less

@@ -17,15 +17,17 @@
 .umb-button-group {
 
     .umb-button__button {
-        border-radius: @baseBorderRadius;
+        border-radius: @baseBorderRadius 0 0 @baseBorderRadius;
+
+        &:hover {
+            z-index: 2;
+        }
     }
 
     .umb-button-group__toggle {
         border-radius: 0 @baseBorderRadius @baseBorderRadius 0;
-        border-left: 1px solid rgba(0,0,0,0.09);
-        margin-left: -2px;
+        margin-left: -1px;
         padding-left: 10px;
         padding-right: 10px;
     }
-
 }