Bug 1932308 - Do not send PRIORITY_UPDATE H2 frames a=pascalc

As far as I can tell, this problem occurs regardless of what is
in the PRIORITY_UPDATE frames. I suspect the connection gets closed by
the server because it's receiving a lot of HTTP/2 frames it does not
understand, and treats these as a DoS attack.

I've also tried to convince Chrome to generate a PRIORITY_UPDATE frame
but I was unsuccessful.

My suggestion is to disable sending PRIORITY_UPDATE for now.

Original Revision: https://phabricator.services.mozilla.com/D230929

Differential Revision: https://phabricator.services.mozilla.com/D231065

modules/libpref/init/StaticPrefList.yaml

@@ -13946,7 +13946,7 @@
 # PRIORITY_UPDATE frames.
 - name: network.http.http2.priority_updates
   type: RelaxedAtomicBool
-  value: true
+  value: false
   mirror: always
 
 # This pref controls whether to send the