Fully specify ingressClassName

As per
[here](https://kubernetes.io/docs/concepts/services-networking/ingress/#deprecated-annotation),
the annotation-based approach of setting Ingress Class was deprecated in
Kubernetes v1.18, which is presumably why there is a [check](7deb6fb920/helm/oncall/templates/ingress-regular.yaml (L4))
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.

helm/oncall/templates/ingress-regular.yaml

@@ -23,7 +23,10 @@ metadata:
     {{- toYaml . | nindent 4 }}
   {{- end }}
 spec:
-  {{- if .Values.ingress.tls }} 
+  {{- if and .Values.ingress.className (semverCompare ">=1.18-0" .Capabilities.KubeVersion.GitVersion) }}
+  ingressClassName: {{ .Values.ingress.className }}
+  {{- end}}
+  {{- if .Values.ingress.tls }}
   tls:
       {{- tpl (toYaml .Values.ingress.tls) . | nindent 4 }}
   {{- end }}