Fully specify ingressClassName

[scubbo]

As per here, the annotation-based approach of setting Ingress Class was deprecated in Kubernetes v1.18, which is presumably why there is a check to only add the annotation for Kubernetes versions lower than this. However, there was no corresponding logic to add an explicit ingressClassName for later versions. This change adds that.

[scubbo]

NOTE that .Capabilities.KubeVersion.GitVersion is deprecated as of Helm 3.x. The usages of [...].GitVersion in this file should probably be replaced with [...].Version (or, for full compatibility, declare a variable like:

{{- if (semverCompare ">=3.x" .Capabilities.HelmVersion.Version) -}}
{{- $kube_version := .Capabilities.KubeVersion.Version -}}
{{- else -}}
{{- $kube_version := .Capabilities.KubeVersion.Version -}}
{{- end -}}

) - but, since I'm extremely new to Helm charts, I wanted to keep this commit as small as possible to minimize the chances of me misunderstanding something or making a mistake. If that's the correct change to make (or if someone can point me in the right direction), I'd be happy to make that change too.

[iskhakov]

I agree with #567 (comment), changing GitVersion can be done in the different PR