Upgrade instructions for when Traefik or Dask Gateway's CRDs has changed

[consideRatio]

Any Helm chart that provides CRD resources, as we do, should have a clear procedure related to it because they are treated differently than other resources in a Helm chart.

The topic is quite complicated, but here are relevant resources:

• Helm's official documentation making a note that they won't upgrade CRDs once they are installed: https://helm.sh/docs/chart_best_practices/custom_resource_definitions/#some-caveats-and-explanations
• Helm's summary about the complexity of managing CRDs: https://github.com/helm/community/blob/main/hips/hip-0011.md
• About Traefik's Helm chart not providing upgrade instructions following changes to CRDs: Establish CRD update process traefik/traefik-helm-chart#581
• About Prometheus' Helm chart that does provide upgrade instructions when CRDs has changed: https://github.com/prometheus-community/helm-charts/tree/main/charts/kube-prometheus-stack#upgrading-an-existing-release-to-a-new-major-version

I've only skimmed through this currently but can conclude that our chart upgrades are failing because the modern Traefik version we now use in the Helm chart assumes updated CRDs are installed. The CRDs aren't modern though, because helm won't upgrade them as part of upgrading the chart if they are already around.

To not block the release process fully, my plan is to investigate if the instructions Prometheus provides about manually applying the CRDs ahead of time is sufficient for us as well.

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The following upgrade instructions has been found to be functional!

# Helm chart users need to run the following when upgrading to 2022.4.0 from 0.9.0
kubectl apply -f https://raw.githubusercontent.com/dask/dask-gateway/2022.4.0/resources/helm/dask-gateway/crds/daskclusters.yaml
kubectl apply -f https://raw.githubusercontent.com/dask/dask-gateway/2022.4.0/resources/helm/dask-gateway/crds/traefik.yaml