Disable http2 support for now

Issue: #7610

src/libsync/accessmanager.cpp

@@ -97,16 +97,10 @@ QNetworkReply *AccessManager::createRequest(QNetworkAccessManager::Operation op,
 #if QT_VERSION >= QT_VERSION_CHECK(5, 9, 4)
     // only enable HTTP2 with Qt 5.9.4 because old Qt have too many bugs (e.g. QTBUG-64359 is fixed in >= Qt 5.9.4)
     if (newRequest.url().scheme() == "https") { // Not for "http": QTBUG-61397
-        // Qt 5.12.4 fixed QTBUG-73947 - http2 should be usable after that
-        bool http2Allowed = QLibraryInfo::version() >= QVersionNumber(5, 12, 4);
+        // http2 seems to cause issues, as with our recommended server setup we don't support http2, disable it by default for now
+        static const bool http2EnabledEnv = qEnvironmentVariableIntValue("OWNCLOUD_HTTP2_ENABLED") == 1;
 
-        static auto http2EnabledEnv = qgetenv("OWNCLOUD_HTTP2_ENABLED");
-        if (http2EnabledEnv == "1")
-            http2Allowed = true;
-        if (http2EnabledEnv == "0")
-            http2Allowed = false;
-
-        newRequest.setAttribute(QNetworkRequest::HTTP2AllowedAttribute, http2Allowed);
+        newRequest.setAttribute(QNetworkRequest::HTTP2AllowedAttribute, http2EnabledEnv);
     }
 #endif