fixes #11139: add position: relative; to .modal-dialog so that z-index takes effect

dist/css/bootstrap.css

@@ -5933,6 +5933,7 @@ button.close {
 }
 
 .modal-dialog {
+  position: relative;
   z-index: 1050;
   width: auto;
   padding: 10px;

less/modals.less

@@ -34,6 +34,7 @@
 
 // Shell div to position the modal with bottom padding
 .modal-dialog {
+  position: relative;
   margin-left: auto;
   margin-right: auto;
   width: auto;