fix(config,debian) fix multistream backwards compatibility

Fixes: https://github.com/jitsi/jitsi-meet/issues/12788

config.js

@@ -1544,6 +1544,12 @@ var config = {
     // },
 };
 
+// Temporary backwards compatibility with old mobile clients.
+config.flags = config.flags || {};
+config.flags.sourceNameSignaling = true;
+config.flags.sendMultipleVideoStreams = true;
+config.flags.receiveMultipleVideoStreams = true;
+
 // Set the default values for JaaS customers
 if (enableJaaS) {
     config.dialInNumbersUrl = 'https://conference-mapper.jitsi.net/v1/access/dids';

debian/jitsi-meet-web-config.postinst

@@ -176,9 +176,10 @@ case "$1" in
         fi
 
         # Fixes multi-stream flags to workaround problem with mobile joining a multi-stream call with multi-stream disabled
-        FIX_MSG="//Enables multi-stream, do not delete me"
+        FIX_MSG="// Temporary backwards compatibility with old mobile clients."
         if ! grep -q "^${FIX_MSG}" $JITSI_MEET_CONFIG; then
-            sed -i "s#config.flags.sourceNameSignaling#${FIX_MSG}\nconfig.flags = config.flags || {};\nconfig.flags.sourceNameSignaling#g" $JITSI_MEET_CONFIG
+            echo  $FIX_MSG >> $JITSI_MEET_CONFIG
+            echo "config.flags = config.flags || {};" >> $JITSI_MEET_CONFIG
         fi
         if ! grep -q "^config.flags.sourceNameSignaling*" $JITSI_MEET_CONFIG; then
             echo "config.flags.sourceNameSignaling = true;" >> $JITSI_MEET_CONFIG