It appears that the version of nginx-ingress we are using (1.34.3) is a) super old and b) is now being pulled from a deprecated chart repository c) is using the beta ingress API that we want to move away from so we can upgrade to kubernetes 1.22
The new official location of this appears to be: https://github.com/kubernetes/ingress-nginx/tree/main/charts/ingress-nginx / https://artifacthub.io/packages/helm/ingress-nginx/ingress-nginx (as per https://github.com/helm/charts/tree/master/stable/nginx-ingress)
As per the documentation at https://github.com/kubernetes/ingress-nginx/blob/main/charts/ingress-nginx/CHANGELOG.md it seems that 4.2.5 is the newest chart (with controller version v1.3.1) that supports Kubernetes 1.21. So we'll need to update to this point first.