fix: Unable to use permission grid dropdowns due to z-index mistake (#3065)

The one I had suggested here: https://github.com/flarum/core/pull/2887#pullrequestreview-690047538

framework/core/less/admin/PermissionsPage.less

@@ -78,7 +78,7 @@
       position: sticky;
       left: 0;
       padding-right: 50px;
-      z-index: 2;
+      z-index: 4;
       background: inherit;
 
       .icon {
@@ -143,7 +143,6 @@
 
   td, th {
     position: relative;
-    z-index: 0;
   }
   th {
     font-weight: normal;