Bug 1735595 - Do not cache 408 responses r=necko-reviewers,kershaw

Facebook responds with 408 when a HTTP/3 connection has been idle for too
long. This is problematic since there are no caching headers added to the
response and it causes application errors if the response is loaded from
the cache.

Differential Revision: https://phabricator.services.mozilla.com/D136116

netwerk/protocol/http/nsHttpChannel.cpp

@@ -2376,9 +2376,10 @@ nsresult nsHttpChannel::ContinueProcessResponse3(nsresult rv) {
       }
       break;
 
+    case 408:
     case 425:
     case 429:
-      // Do not cache 425 and 429.
+      // Do not cache 408, 425 and 429.
       CloseCacheEntry(false);
       [[fallthrough]];  // process normally
     default: