Make UHF popup a modal (#8261)

Author: anna-dingler

source/nodejs/adaptivecards-site/themes/adaptivecards/layout/_partial/header.ejs

@@ -156,7 +156,9 @@
 															data-m='{"cN":"GlobalNav_More_nonnav","id":"nn1c1c9c2m1r1a1","sN":1,"aN":"c1c9c2m1r1a1"}'>
 															<span>All Microsoft</span>
 														</button>
-														<ul class="f-multi-column f-multi-column-6"
+														<ul role="dialog"
+															aria-modal="true"
+															class="f-multi-column f-multi-column-6"
 															aria-hidden="true"
 															data-m='{"cN":"More_cont","cT":"Container","id":"c2c1c9c2m1r1a1","sN":2,"aN":"c1c9c2m1r1a1"}'>
 															<li class="c-w0-contr">