Clarify actual expected version for kubernetes and Ingress across docs and helm chart

[pragmaticivan]

Describe the feature

Here it requires 1.16 https://docs.flagger.app/tutorials/linkerd-progressive-delivery#prerequisites Here it requires 1.19 https://docs.flagger.app/tutorials/nginx-progressive-delivery#prerequisites Here it requires 1.16 https://github.com/fluxcd/flagger/blob/main/charts/flagger/Chart.yaml#L5 This tutorial still uses v1beta1 https://docs.flagger.app/tutorials/linkerd-progressive-delivery#linkerd-ingress

My guess here is that for using with nginx ingress only, 1.19 is required.

I keep getting this error:

Warning  Synced  18s (x2 over 78s)    flagger  ingress canary-playground.shared get query error: the server could not find the requested resource

Probably because my ingress is still using this version:

apiVersion: extensions/v1beta1
kind: Ingress

Very likely to be related to https://github.com/fluxcd/flagger/issues/956

Proposed solution

Support ingress version while using kubernetes 1.16.

Note: I'm currently using 1.18, which does not have this resource version:

apiVersion: networking.k8s.io/v1
kind: Ingress

[pragmaticivan]

Context: https://github.com/fluxcd/flagger/blob/main/pkg/router/ingress.go#L51

I think by getting apiVersion from ingressRef it's possible to conditionally support both.