Bump go version for trivy ci to 1.21

[coderzc]

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This change is a trivial rework / code cleanup without any test coverage.

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[github-actions]

@coderzc:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)