Remove documentation for removed in-tree openstack provider

[LarssonOliver]

What type of PR is this?
/kind documentation

What this PR does / why we need it:

The in-tree cloud provider was removed in Kubernetes 1.26. This PR simplifies the documentation by removing instructions related to this removed provider.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Remove documentation for removed in-tree openstack provider

[k8s-ci-robot]

Hi @LarssonOliver. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[yankay]

/ok-to-test

[yankay]

Thanks @LarssonOliver
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[VannTen]

/lgtm