Suggest calico_iptables_backend: "NFT" In Centos8

[yankay]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
The calico_iptables_backend=Auto is not stable. The problem is very often in Centos 8 , Oracle Linux 8 and RHEL 8.
So should we suggest user to use calico_iptables_backend: "NFT" instead of the "auto" mode, only suggest user to use the calico_iptables_backend: "NFT".

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
NONE

Does this PR introduce a user-facing change?:
NONE

Reasons:

1. RHEL/Centos 8 ships a hacked version of iptables 1.8 that only supports nft mode from the (https://github.com/kubernetes-sigs/iptables-wrappers"),
2. We failed on Calico in Centos8 many time. The "auto" mode is not stable after the cluster runned for days.
3. The docs of RHEL in kubespray : https://github.com/kubernetes-sigs/kubespray/blob/master/docs/rhel.md is NFT mode
4. a blog article about the issue: https://mihail-milev.medium.com/no-pod-to-pod-communication-on-centos-8-kubernetes-with-calico-56d694d2a6f4

Issues Relate to the PR:

• Calico autoselects iptables-legacy mode if ip_tables.ko preloaded on CentOS 8 projectcalico/calico#3709
• Calico networking broken when host OS uses iptables >= 1.8 projectcalico/calico#2322
• Networking broken on CentOS/OracleLinux 8.3 #7268

[k8s-ci-robot]

Hi @yankay. Thanks for your PR.

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[oomichi]

/ok-to-test
/lgtm

[floryut]

@yankay Thank you 👍

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, yankay

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[weizhoublue]

great idea, I encountered same issue