Instructions for `pkgs.k8s.io` should be updated to check GPG fingerprint

https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/install-kubeadm/
https://kubernetes.io/blog/2023/08/15/pkgs-k8s-io-introduction/

The current documentations instruct to trust `https://pkgs.k8s.io/core:/stable:/v1.28/deb/Release.key` without checking its fingerprint:

https://github.com/kubernetes/website/blob/833291687fd2a241efd3cb295dc4973af3696942/content/en/docs/setup/production-environment/tools/kubeadm/install-kubeadm.md?plain=1#L181-L192

This doesn't seem to be more secure than just relying on HTTPS.
The documentation should be updated to check its fingerprint.
Or, the GPG can be just removed if the HTTPS infrastructure for `https://pkgs.k8s.io` is considered to be trustable.

Start with content/en/docs/setup/production-environment/tools/kubeadm/install-kubeadm.md, especially the instructions around lines 181-192, and review the linked pkgs.k8s.io introduction. Determine how the documentation should address verification of Release.key or its removal. Done means the installation guidance clearly reflects the chosen trust and verification approach.

Written by the indexing model from the issue text.