update Azure compute API to 2021-04-01

[jackfrancis]

What type of PR is this?

What this PR does / why we need it:

This PR updates the Azure compute API Version to 2021-04-01 (from 2020-06-30). In the large, we simply fulfill any necessary interface changes that the newer API Version requires (primarily changing const references as needed).

There are two specific interface changes that are interesting:

• VMSS client Get includes a new expand option. We pass in "" to indicate that we are not using that interface feature
  • https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2021-04-01/compute#VirtualMachineScaleSetsClient.Get
• VMSS client Delete includes a new "force delete" option. We pass in *false to bypass this new, preview (as of this new API version) feature
  • https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2021-04-01/compute#VirtualMachineScaleSetsClient.Delete

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 #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

update Azure compute API to 2021-04-01

[devigned]

/lgtm
/approve

Even though this change includes several files, the heart of the change is relatively benign.

[k8s-ci-robot]

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