Deprecate support for RedHat 8 family hosts

[VannTen]

RHEL 8 (and derivates, like Almalinux 8, Rockylinux 8, and others) are starting to cause some problems:

• by default, they ship a kernel version which is below the ones recommended by kubeadm : [kubernetes] Support Kubernetes v1.32.0 #11862 (comment)
• the default python version is not supported by ansible-core 2.17 (and thus ansible 10) : Upgrade ansible from 9 to 10 (ansible-core from 2.16 to 2.17) #11519 (comment)
• (minor): they come with an old systemd version (v239) which would need some tweaks with containerd 2.0 for instance : [containerd] Support containerd v2.0.x #11845 (comment)

Another piece of information to consider is that the security support for RHEL8 and clones (alma, rocky) ends in 2029. Given the pace of kubernetes development, I don't think we will be able to support them until that date (cgroupsv2 is not mandated for now, but it might happen,)

So, this is to kick-off a discussion about a timeline to deprecate, and eventually remove, support for RHEL8 family (and also: Opensuse 15 I think ? -> in general OS with kernel < 5.8/10 and python < 3.7)

I see the following extreme possiblities:

• wait until 2029
• deprecate now as part of defaulting to K8s 1.32, remove when 1.31 is removed from our options (== 2.30 I think ?)

There is problably some room between those.

Slack thread on that subject on what other sig cluster-lifecycle projects are doing: https://kubernetes.slack.com/archives/C13J86Z63/p1736414039059959

@ant31 @floryut @yankay @tico88612 @mzaian

Others, feel free to chime in. I'm going to pin this issue for visibility.

[ant31]

Let's drop them and remove from CI.

Eventually we do as kops, have a flag to turn off preflight check with big warning -> close all future issues raised about it.

Not knowing what Kubespray users are using doesn't help.

[VannTen]

Yeah, that's basically why I meant by deprecation.
The removal would be: removing stuff from boostrap-os, preinstall, etc, stuff we know will definitely break on those distribs.

We already have the turn off prefligt, (with a variable) since #11710

[ant31]

let's go for it (removing support).
RHEL 8 and fam., can request to RH and other about bumping kernel in LTS or move on with newer version.