[Feature]: clean docs about CSI

[guettli]

Contact Details

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Is there an existing issue for this?

• [X] I have searched all the existing issues

Is your feature request related to a problem? Please describe.

https://cubefs.io/docs/master/user-guide/k8s.html

I think it's time to remove docs about Kubernetes versions before v1.13

Describe the solution you'd like.

I didn't want to see outdated docs

Describe an alternate solution.

No response

Anything else? (Additional Context)

No response

[xiaochunhe]

@heymingwei please consider this suggestion

[heymingwei]

The k8s version used by many enterprises is relatively old, so this part of the content is still valuable and should be retained. @guettli