Update Dockerfile to Go 1.20

[mochizuki875]

What this PR does / why we need it:

In #257, Go version has been updated 1.20 from 1.17.
However the Go version of Dockerfile was still 1.17, so I fixed it.

Tested:

Unit and E2E tests has passed.

[k8s-ci-robot]

Hi @mochizuki875. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mochizuki875]

/assign @adrianludwin

[adrianludwin]

Whoops, good catch!

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adrianludwin, mochizuki875

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