Update e2e K8s test version matrix

[bwagner5]

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K8s 1.24 was released for EKS and kOps already support 1.25.

The e2e K8s version matrix can be updated to K8s 1.21, 1.22, 1.23, 1.24, and 1.25.

K8s 1.18-1.20 is now EOL in EKS and has been EOLd upstream for quite a while. https://docs.aws.amazon.com/eks/latest/userguide/kubernetes-versions.html

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[snay2]

Agreed. We're planning to make this change once the PSP deprecation is complete, to enable support for 1.25. Tracking that work here: #638

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[cjerad]

Included in the v1.19.0 release of AWS Node Termination Handler, Helm chart v0.21.0