Update clearfix mixin (#21224)

* Update clearfix to use block instead of table display (also reorder properties for linting)
* update docs snippet for clearfix mixin—was apparently still using Less syntax and had old clearfix hack (even before the block change in this PR)

scss/mixins/_clearfix.scss

@@ -1,7 +1,7 @@
 @mixin clearfix() {
   &::after {
+    display: block;
     content: "";
-    display: table;
     clear: both;
   }
 }