vxlan: T5671: change port to IANA assigned default port

Currently VyOS VXLAN implementation uses the Linux assigned port 8472 that
predates the IANA assignment. As Most other vendors use the IANA assigned port,
follow this guideline and use the new default port 4789.

Existing configuration not defining an explicit port number will be migrated
to the old default port number of 8472, keeping existing configurations work!