look for SHA256SUMS instead of MD5SUMS, which is not published anymore

the code still calls everything "md5" but it's not md5 anymore, that
should be updated everywhere.
Signed-off-by: Mattia Rizzolo <mapreri@ubuntu.com>

uitwww/download.py

@@ -118,7 +118,7 @@ class Downloads:
                     if path not in files_content:
                         md5s = {}
 
-                        response = requests.get("%s/MD5SUMS" % path)
+                        response = requests.get("%s/SHA256SUMS" % path)
                         if response.status_code == 404:
                             raise RuntimeError(
                                 "Missing {} {} on {}".format(distro, release, url),