remove experimental note about CentOS 8 and derivatives

[cristicalin]

What type of PR is this?

/kind documentation

What this PR does / why we need it:
Since CentOS 8 is relatively well tested now we should remove the experimental flag for the 2.16 release (see #7602).

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Support for CentOS 8 and derivatives is considered stable

[k8s-ci-robot]

Hi @cristicalin. Thanks for your PR.

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[floryut]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[oomichi]

/lgtm

[oomichi]

I am using Kubespray heavily on CentOs8 and RHEL8.
I am not against that we can say that is well tested.

/lgtm