fix: stage conformance/go.sum in release helper

[danehans]

What type of PR is this?
/kind bug

What this PR does / why we need it:
This updates the release helper to stage conformance/go.sum alongside conformance/go.mod after the conformance module dependency is updated during make release.

Without this, the release workflow can leave checksum changes unstaged and produce an incomplete release commit.

Which issue(s) this PR fixes:
Fixes #2803

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danehans

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• 949af22 fix: stage conformance/go.sum in release helper

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[ahg-g]

/lgtm