chore(docs): remove stale supported version info

[cwrau]

What this PR does / why we need it:

Sometimes it's better not to have documentation than having outdated docs. Otherwise people see the docs, think they are up-to-date and wait for updates.

Special notes for your reviewer:

1. See https://kubernetes.slack.com/archives/CFKJB65G9/p1714665937168969?thread_ts=1694426926.778949&cid=CFKJB65G9

/hold

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[k8s-ci-robot]

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[mdbooth]

/ok-to-test /lgtm

@lentzi90 what do you think?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lentzi90

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/main/OWNERS)~~ [lentzi90] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[EmilienM]

/hold cancel