Add support for IngressClassName

[christiangonre]

Description

I was trying to set up the ingress for Posthog and I saw that this option wasn´t available in the helm chart, that´s why I decided to push this PR. The ingressClassName is the recommended way to specify what Ingress controller do you want to use since Kubernetes > 1.18.

See this URL for more info

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

To test this implementation I deployed locally Posthog and checked that the ingress was working fine with the new ingressClassName properly set in the ingress resource.
In case you use the annotation kubernetes.io/ingress.class: nginx for speciying the ingress controller that you want to use, you can delete it and add this line under ingress:
ingressClassName: nginx

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works