Increase browser version requirements

Now that we use ResizeObserver we know that we require more modern
browsers. The most notable ones here are Firefox and Safari.

With regards to Firefox, while the desktop version has had support
since 69, the Android app requires 79. At the time of writing the
current ESR of Firefox is 78, but the concept of ESR doesn't seem to
exist for Android.

The Safari 13 requirement means we no longer support for example iPhone
5S or the 4th generation of the iPad. These are devices from 2013~2014.

README.md

@@ -91,7 +91,7 @@ noVNC uses many modern web technologies so a formal requirement list is
 not available. However these are the minimum versions we are currently
 aware of:
 
-* Chrome 49, Firefox 44, Safari 11, Opera 36, Edge 79
+* Chrome 64, Firefox 79, Safari 13.4, Opera 51, Edge 79
 
 
 ### Server Requirements