Update supported browser versions (#15207)

Mention Safari 8 and iOS 8 and bump evergreen browser versions

Change-Id: I9c6494a0e5967539385fabd05b540e1f0622b96c

WebContent/release-notes.html

         </p>
 
         <ul>
-            <li>Mozilla Firefox 18-29</li>
-            <li>Mozilla Firefox 17 ESR, 24 ESR</li>
+            <li>Mozilla Firefox 18-33</li>
+            <li>Mozilla Firefox 17 ESR, 24 ESR, 31 ESR</li>
             <li>Internet Explorer 8-11</li>
-            <li>Safari 6-7</li>
-            <li>Opera 12, 16-20</li>
-            <li>Google Chrome 23-34</li>
+            <li>Safari 6-8</li>
+            <li>Opera 12, 16-25</li>
+            <li>Google Chrome 23-38</li>
         </ul>
 
         <p>
         </p>
 
         <ul>
-            <li>iOS 5-7</li>
+            <li>iOS 5-8</li>
             <li>Android 2.3-4</li>
             <li>Windows Phone 8</li>
         </ul>