🌱 Rename `v1alpha8` to `v1beta1`

[EmilienM]

What this PR does / why we need it:

We are getting close to releasing our first stable API via v1beta1. We agreed that we would rename v1alpha8 to v1beta1.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1901

/hold

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: EmilienM, lentzi90

The full list of commands accepted by this bot can be found here.

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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

[lentzi90]

/test pull-cluster-api-provider-openstack-e2e-full-test

[EmilienM]

/test pull-cluster-api-provider-openstack-e2e-full-test

@mdbooth are you ok to do it now? If not I'll stop rebasing because I think it's going to merge conflict quite often.

[mdbooth]

Yeah, I think we're confident we're doing this.

/lgtm

[EmilienM]

/hold cancel