Fix UPF crash at startup after latest Docker and Ubuntu updates

doc/tutorial_resources/oai-cn5g/conf/config.yaml

@@ -211,7 +211,7 @@ upf:
   support_features:
     enable_bpf_datapath: no    # If "on": BPF is used as datapath else simpleswitch is used, DEFAULT= off
     enable_snat: yes           # If "on": Source natting is done for UE, DEFAULT= off
-  remote_n6_gw: localhost      # Dummy host since simple-switch does not use N6 GW
+  remote_n6_gw: 127.0.0.1      # Dummy host since simple-switch does not use N6 GW
   smfs:
     - host: oai-smf            # To be used for PFCP association in case of no-NRF
   upf_info: