Drop support for k8s 1.21

[consideRatio]

• Closes In March 2023 - drop support for k8s 1.21 #2979 as GKE/EKS/AKS has dropped support for k8s 1.21 now a bit earlier than March as previously declared.

I squeezed in a few smaller docs updates into this PR that could have been separate PRs. The linkcheck failure is unrelated and reported in jupyterhub/jupyterhub#4376.

[consideRatio]

We have an upgrade test using k8s 1.22 still defined below.

[manics]

Can we drop 1.22 as well? May 2023 seems close enough, and it sets expectations now.

[consideRatio]

Can we drop 1.22 as well? May 2023 seems close enough, and it sets expectations now.

I'd like to do so too and get z2jh 3.0.0 with jupyterhub 4 together with releases of kubespawner/oauthenticator out.

I think maybe I can help better testing the existing dev branches with jupyterhub 4 if we don't remove k8s 1.22 support just before we must. Due to that, and due to I already have a PR ready to push open to drop 1.22 support I'd like to have that be a separate PR.

• Fully agree on the idea to drop 1.22 before next major release though - see In May 2023 (or before z2jh 3.0.0 release?) - drop support for k8s 1.22 #3040.

[manics]

If you're not dropping 1.22 immediately can you update https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/main/docs/source/changelog.md#unreleased-breaking-changes ?

[consideRatio]

If you're not dropping 1.22 immediately can you update https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/main/docs/source/changelog.md#unreleased-breaking-changes ?

Done already!

I'll update it again in the next pr when we require k8s 1.23+!