Remove the concept of extended fields from our standard and experimental channels

[tssurya]

Describe the issue:

We have 2 channels:

1. standard channel signifies the fields are stable and all implementations must implement all fields in this channel -> also called core API fields
2. experimental channel signifies standard channel plus extra new fields in addition to the core fields that are not yet stable and are waiting for implementations to implement before they can become part of the core. However these fields were called extended fields and had another notion of "optional to implement" for implementations

We have spoken in past sig-network-policy-api meetings and at kubecon that community is OK to get rid of these extended/core concept totally and just have the two channels -> All implementations that are implementing experimental must implement all the fields there and same goes for core.

[tssurya]

/assign @tssurya

[danwinship]

We should also consider whether the "channel" terminology makes sense. (It seems to not really be working well for Gateway anyway?)

We just have "experimental features", right?

[danwinship]

We should also consider whether the "channel" terminology makes sense.

(maybe after the next release)