Require py39, k8s 1.24, use golang 1.21, update traefik from v2.9 to v2.10

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• Non-breaking: use golang 1.21 to compile dask-gateway-proxy
• Dropping support for Python 3.8 as that is what dask/distributed and other packages has done already in the dask ecosystem.
• Dropping support for k8s 1.23 and lower. For related discussions about things like this, see In Oct 2023 - drop support for k8s 1.23 jupyterhub/zero-to-jupyterhub-k8s#3042.
• Upgrading the helm chart's traefik version from v2.9 to v2.10, which forces users upgrading the helm chart to also upgrade bundled CRD resources. A note is to be made in the changelog about doing kubectl apply before ugprading as has been done for previous releases when this has become needed.

[consideRatio]

Thanks for reviewing @TomAugspurger!

I did a delayed self-review and realized a possible issue for existing clusters, but after reviewing it further I don't think its an issue - I believe existing clusters will continue to work well even if the dask-gateway helm chart is upgraded.

By adopting the some new stuff in traefik v2.10, the upgrade to traefik v3 at some time in the future will be less disruptive though - as clusters created with traefik v2.9 (but not v2.10) would need to be re-created before traefik v3 is used.

Without objections by tomorrow, I'll go for a merge and cut a release. As can be seen from f2e0e48, its not much that we've done, but there are some bugfixes and a new feature for the helm chart.