Remove high availability yaml for < 1.21

[rittneje]

metrics-server has not supported a Kubernetes version less than 1.21 since v0.6. Consequently, there does not seem to be any need to have both high-availability-1.21+.yaml and high-availability.yaml release assets anymore.

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[Dean-Coakley]

I would be open to sending a PR for this.

@dgrisonnet would you be open to accepting this change? Would you also like any changes to the compatibility matrix https://github.com/kubernetes-sigs/metrics-server/tree/master?tab=readme-ov-file#compatibility-matrix Maybe it just makes sense to include the latest 3 releases or so?

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/lifecycle stale

[rittneje]

/remove-lifecycle stale