deploy: remove fake routes from GnB's NetworkAttachmentDefinition in free5gc deployments

[PapaySail]

• fixed problem: UE connects and disconnects The reason of "signal lost" is a fake default gateways in GnB's deployment config Solution is to remove , "gateway": [ "10.100.50.nnn" ] The problem's root is in towards5gs helm chart To overcome this issue we use post-render kustomize script which overrides NetworkAttachmentDefinition before helm installed.

• fixed free5gculcl eUPF: add gtp_peer to config. eUPF have to send GTP Echo Request to every known GTP peer., according to #465

[pirog-spb]

@PapaySail Is it possible just to configure default route properly in values file?

[pirog-spb]

Please fix PR name in order to match conventional commit style