v11: Fix for TinyMCE dropdowns (#13417)

* Set higher z-index for TinyMCE overlays as they were set below that of our property editor overlays

Fixes #13414

* replace !important with more specificity

src/Umbraco.Web.UI.Client/src/less/rte.less

@@ -140,6 +140,12 @@
     }
 }
 
+.tox {
+  &.tox-tinymce-aux {
+    z-index: 65535;
+  }
+}
+
 .tox-tinymce-inline {
   z-index: 999;
 }