🐛 bump go version for container images to 1.21

[schrej]

What this PR does / why we need it:
Bumps the Go version used to build container images to 1.21

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 #

/lgtm
/approve

[k8s-ci-robot]

@schrej: you cannot LGTM your own PR.

In response to this:

What this PR does / why we need it:
Bumps the Go version used to build container images to 1.21

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 #

/lgtm
/approve

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: schrej

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• OWNERS [schrej]

Approvers can indicate their approval by writing /approve in a comment
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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: schrej

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [schrej]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment