azure scalability: use latest CAPZ release

[jackfrancis]

This PR updates the version of CAPZ that we use to build SIG Scalability cluster scenarios uses the latest release branch of CAPZ following the release of v1.21.0:

• https://github.com/kubernetes-sigs/cluster-api-provider-azure/releases/tag/v1.21.0

[jackfrancis]

/assign @wojtek-t @mm4tt @mborsz @marseel

[jackfrancis]

gentle ping @wojtek-t @mm4tt @mborsz @marseel

[nojnhuh]

This job is currently failing and this change should fix it. From #35893

The CAPZ release-1.20 branch is currently failing to build clusters for the ci-kubernetes-e2e-azure-scalability job since the removal of the kubelet's --pod-infra-container-image flag in kubernetes/kubernetes#133779, e.g. https://prow.k8s.io/view/gs/kubernetes-ci-logs/logs/ci-kubernetes-e2e-azure-scalability/1986706943242145792.

This PR bumps CAPZ to the release-1.21 branch which includes kubernetes-sigs/cluster-api-provider-azure#5961 to reference newer VM images built with kubernetes-sigs/image-builder#1866 that no longer set that flag.

[nojnhuh]

/lgtm

[wojtek-t]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, nojnhuh, wojtek-t

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• config/jobs/kubernetes/sig-scalability/OWNERS [wojtek-t]

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[k8s-ci-robot]

@jackfrancis: Updated the job-config configmap in namespace default at cluster test-infra-trusted using the following files:

• key sig-scalability-periodic-azure.yaml using file config/jobs/kubernetes/sig-scalability/sig-scalability-periodic-azure.yaml

In response to this:

This PR updates the version of CAPZ that we use to build SIG Scalability cluster scenarios uses the latest release branch of CAPZ following the release of v1.21.0:

• https://github.com/kubernetes-sigs/cluster-api-provider-azure/releases/tag/v1.21.0

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