KEP-4794: initial proposal for deprecating v1.Endpoints

[danwinship]

The `v1.Endpoints` API has been essentially deprecated since
EndpointSlices became GA in 1.21. Several new Service features (such
as dual-stack and topology, not to mention "services with more than
1000 endpoints") are implemented only for EndpointSlice, not for
Endpoints. Kube-proxy no longer uses Endpoints ever, for anything, and
the Gateway API conformance tests also require implementations to use
EndpointSlices.

Despite this, kube-controller-manager still does all of the work of
managing Endpoints objects for all Services, and a cluster cannot pass
the conformance test suite unless the Endpoints and EndpointSlice
Mirroring controllers are running, even though in many cases nothing
will ever look at the output of the Endpoints controller (and the
EndpointSlice Mirroring controller will never output anything).

While Kubernetes's API guarantees make it essentially impossible to
ever actually fully remove Endpoints, we should at least move toward a
world where most users run Kubernetes with the Endpoints and
EndpointSlice Mirroring controllers disabled.

#4974

/sig network
/cc @aojea @robscott

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