✨ Add worker version pinning API and validation

[AcidLeroy]

What this PR does / why we need it:

First of three PRs implementing worker version pinning (CAEP #13892). Adds an optional version field to MachineDeploymentTopology/MachinePoolTopology behind the new ClusterTopologyWorkerVersionPinning feature gate (alpha, off by default), plus admission validation.

API and validation only — the topology controller does not read the field yet, so this is a no-op at runtime. Controller and e2e follow in PRs 2 and 3.

flowchart LR
  A["MD/MP<br/>cluster-managed"] -->|"set version<br/>= running version"| B["pinned<br/>no rollout"]
  B -->|"raise version"| C["pinned<br/>rolls out"]
  C -->|"unset version"| A

  V{"validating<br/>webhook"}
  B -.-> V
  C -.-> V
  V -.- R["no decrease, not below running version<br/>&le; control plane version, within skew policy<br/>unset only when = topology.version<br/>member of ClusterClass.kubernetesVersions<br/>not with defer-upgrade / hold-upgrade-sequence"]

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Cluster.spec.topology.version is validated against every pinned version too, so the Cluster cannot outrun a pinned MD/MP's skew budget. This is the only such guard for chained upgrades, where the +2 minor ceiling is skipped entirely.

Also fixes a latent bug: validateTopologyMachineDeploymentVersions/validateTopologyMachinePoolVersions treat any MD/MP whose version differs from topology.version as "still completing a previous upgrade". Without this fix one pinned MD would block every future topology version change.

Notes for reviewers:

• v1beta1 conversion drops the field, so it is restored from the conversion-data annotation matched by topology name, not index — v1beta1 clients can reorder the list.
• Skew rules moved to util/version, shared with the MachineSet preflight checks. The shared helper also compares the major version and avoids the unsigned underflow the previous inline check had when the control plane minor is below the tolerated skew.

Which issue(s) this PR fixes:

Part of #13967

/area clusterclass
/area api

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[kubernetes-prow]

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