Replace endpoint.ParseHostPort with net.SplitHostPort to fix IPv6 client endpoints

[jpbetz]

Fixes kubernetes/kubernetes#83550

Manually verified fix:

etcd --listen-client-urls="https://[::1]:2379" --advertise-client-urls="https://[::1]:2379" --cert-file=integration/fixtures/server-ipv6.crt --key-file=integration/fixtures/server-ipv6.key.insecure

etcdctl --cacert ~/projects/etcd-io/src/go.etcd.io/etcd/integration/fixtures/server-ipv6.crt --endpoints="https://[::1]:2379" get / --write-out json
{"header":{"cluster_id":14841639068965178418,"member_id":10276657743932975437,"revision":1,"raft_term":5}}

We need a automated test for this too. Since we don't have any IPv6 tests in etcd, we need to enable that test carefully, since it would fail in environments without IPv6 enabled. We will do that as a follow up PR.

Regenerated all the test certs after adding one with an IPv6 IP for the manual test. We'll keep the cert around for when we add the automated test.

cc @liggitt @gyuho @jingyih @wenjiaswe @YoyinZyc @hangxie

[neolit123]

LGTM, thanks for the quick fix.

[gyuho]

lgtm, thanks for the quick fix

[neolit123]

given kubernetes/kubernetes is at 3.3.15 would it be possible to also backport for 3.3?

[wenjiaswe]

lgtm, thanks Joe! +1 on backporting to 3.3, ref: kubernetes/kubernetes#83209 (comment)