changed interface naming style

@defosseu
I have changed interface naming from access, core/sgi-lan to N3 and N6 inside docker-container. Here core doesnt mean interface N4 (UPF-SMF)
Because here core (N6) is used for internet (DN) and sgi-lan (N6) is used for additinal packet processing (firewall, edge gw etc.)

So in a nutshell for docker-compose naming -

   Access => N3 => UPF <-->gNB  
   Core   => N6 => UPF <-->DN    
   CP     => N4 => UPF <-->SMF    

Later we will add more interfaces in docker-compose file for special purposes like -

• One more N6 for FlexCn/Edge computing case
  
• N9 for multiple UPF
  So it is better to clear naming style.