Declare cluster.x-k8s.io/v1beta2 contract support in CRD labels

[poussa]

What this PR does / why we need it:

Adds the cluster.x-k8s.io/v1beta2 contract label to CAPK's infrastructure CRDs
(KubevirtCluster, KubevirtClusterTemplate, KubevirtMachine, KubevirtMachineTemplate) via
config/crd/kustomization.yaml, so that core Cluster API v1.13+ can recognize CAPK as
v1beta2-contract-compatible.

This is a metadata-only change, the same mechanism used to originally add the v1beta1 label:

commonLabels: ... cluster.x-k8s.io/v1beta1: v1alpha1

• cluster.x-k8s.io/v1beta2: v1alpha1

No Go dependency bump, no controller code changes, and no change to the CRD's served/storage
API version (v1alpha1 remains the only served version). CAPK continues to watch and reconcile
v1alpha1 resources exactly as before.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #390

Special notes for your reviewer:

This is intentionally much narrower than the earlier v1beta2 effort in #328, which bumped the
cluster-api Go module to v1.11 and updated CAPK's own condition types — that change broke
users still on CAPI v1.10 and was reverted in #361/#363.

This PR does not touch CAPK's dependencies or code at all; it only adds a contract label so that
core CAPI's Cluster/topology controllers can resolve infrastructureRef/controlPlaneRef
lookups when a Cluster is created with apiVersion: cluster.x-k8s.io/v1beta2. Because the
underlying CRD API version is unchanged, this should be safe for users on any CAPI version,
including those still on v1beta1/v1.10.

Verified locally:

• kustomize build config/crd shows cluster.x-k8s.io/v1beta2: v1alpha1 on all 4 CRDs.
• Applied the patched CRDs to a live management cluster running CAPI v1.13.4 + this CAPK
  controller (v0.11.2, unmodified). Created a scratch Cluster (apiVersion: cluster.x-k8s.io/v1beta2) referencing KubevirtCluster via the new infrastructureRef.apiGroup
  style. The core CAPI controller correctly resolved the contract, set
  ownerReferences[].apiVersion: cluster.x-k8s.io/v1beta2 on the KubevirtCluster, and began
  reconciling normally (no "contract not found"/CRD lookup errors). capk-controller-manager
  remained healthy and unaffected throughout.

Release notes:

Add `cluster.x-k8s.io/v1beta2` contract label to CAPK infrastructure CRDs, enabling Cluster API
v1beta2 Cluster resources to reference KubevirtCluster/KubevirtMachine as infrastructure providers.

[kubernetes-prow]

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[kubernetes-prow]

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[poussa]

This PR is not enough. Need code level changes. Closing (for real now).