fix(aio): make the footer links clickable on all browsers

The footer background (implemented via `footer:after`) had a higher `z-index`
than other footer elements and was obscuring the footer links on certain
browsers (Firefox, Edge, IE), which made them unclickable.
This commit lowers the index of `footer:after`, so that links are clickable on
these browsers.

Fixes #17460

aio/src/styles/1-layouts/_footer.scss

@@ -97,7 +97,7 @@ footer {
 footer::after {
     content: "";
     position: absolute;
-    z-index: 10;
+    z-index: -1;
     top: 0;
     bottom: 0;
     left: 0;