KEP-3515: add new kep kubectl explain openapi v3 upgrade (kubernetes#3516)

* add kubectl explain: openapi v3 upgrade kep

* spruce up wording

* reduce scope and take feedback into account

* fix typos

* fixx out kep yaml

* remove prnumber

what is this used for

* fill in testing plan a bit more

* rename raw to openapiv3

* update with feedback

* fix typo

* fill in version skew strategy

* update toc

* formatting fixes

* remove TODOs

* address feedback

* address feedback

* clarify --output fallback

* spelling

* address feedback about rollout

* add explicit not alpha 2 may be dropped

* add note about openapi v3 data not available

* add GA criterion

* changed feature gate to use environment variable

Author: alexzielenski

keps/prod-readiness/sig-cli/3515.yaml

@@ -0,0 +1,3 @@
+kep-number: 3515
+alpha:
+  approver: "@johnbelamaric"