- kubeadm: removed preflight check for nsenter on Linux nodes

* New kubeadm flag `--ignore-preflight-errors` that enables to decrease severity of each individual error to warning. ([#56072](https://github.com/kubernetes/kubernetes/pull/56072), [@kad](https://github.com/kad))
    * Old flag `--skip-preflight-checks` is marked as deprecated and acts as `--ignore-preflight-errors=all`

([#72214](https://github.com/kubernetes/kubernetes/pull/72214), [@caesarxuchao](https://github.com/caesarxuchao))

- kubeadm
  - `kubeadm alpha preflight` and `kubeadm alpha preflight node` are removed; you can now use `kubeadm join phase preflight` ([#73718](https://github.com/kubernetes/kubernetes/pull/73718), [@fabriziopandini](https://github.com/fabriziopandini))

- Kubeadm: switch the preflight check (called 'Swap') that verifies if swap is enabled on Linux hosts to report a warning instead of an error. This is related to the graduation of the NodeSwap feature gate in the kubelet to Beta and being enabled by default in 1.23 - allows swap support on Linux hosts. In the next release of kubeadm (1.24) the preflight check will be removed, thus we recommend that you stop using it - e.g. via `--ignore-preflight-errors`...

- kubeadm: removed `socat` and `ebtables` from kubeadm preflight checks ([#127151](https://github.com/kubernetes/kubernetes/pull/127151), [@saschagrunert](https://github.com/saschagrunert)) [SIG Cluster Lifecycle]

- Kubeadm: Added support during the preflight check "CreateJob" of "kubeadm upgrade" to check if there are no nodes where a Pod can be scheduled. If there are none, show a warning and skip this preflight check. This can happen in single node clusters where the only node was drained. ([#124503](https://github.com/kubernetes/kubernetes/pull/124503), [@neolit123](https:/...

- Kubeadm: during the preflight check "CreateJob" of "kubeadm upgrade", check if there are no nodes where a Pod can schedule. If there are none, show a warning and skip this preflight check. This can happen in single node clusters where the only node was drained. ([#124570](https://github.com/kubernetes/kubernetes/pull/124570), [@neolit123](https://gi...

* kubeadm will now pull required images during preflight checks if it cannot find them on the system. ([#64105](https://github.com/kubernetes/kubernetes/pull/64105), [@chuckha](https://github.com/chuckha))

- The flag `--skip-preflight-checks` of kubeadm has been removed. Please use `--ignore-preflight-errors` instead. ([#62727](https://github.com/kubernetes/kubernetes/pull/62727), [@xiangpengzhao](https://github.com/xiangpengzhao))

* kubeadm now produces error during preflight checks if swap is enabled. Users, who can setup kubelet to run in unsupported environment with enabled swap, will be able to skip that preflight check. ([#55399](https://github.com/kubernetes/kubernetes/pull/55399), [@kad](https://github.com/kad))