Update go.mod

[alongL]

upgrade gopcua from v0.1.5 to v0.2.6, for there are some bugs in gopcua early version.
For example lost connection etc.

[kubeedge-bot]

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[kubeedge-bot]

Welcome @alongL! It looks like this is your first PR to kubeedge/mappers-go 🎉

[sailorvii]

Thanks for the refine. /lgtm

[fisherxu]

@alongL Please fix the failed DCO test :)

[kubeedge-bot]

@alongL: PR needs rebase.

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