Bump go build version to 1.21

[ConnorJC3]

Bumps the go version from 1.20 to 1.21, needed for latest Kubernetes version which relies on the slices package in the standard library that was made available in 1.21.

Created for and required by kubernetes-csi/external-provisioner#1068

[k8s-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ConnorJC3
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[jsafrane]

/retest

[jsafrane]

/close
I approved #240, even though your PR is older. That one had a release note ;-).

[k8s-ci-robot]

@jsafrane: Closed this PR.

In response to this:

/close
I approved #240, even though your PR is older. That one had a release note ;-).

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.