Rename Workhorse go module to gitlab/workhorse

What does this MR do?

GoSec gets confused by the current unconventional Go module layout of Workhorse. If we rename the Go module of Workhorse to gitlab/workhorse, then we are using the normally expected name and GoSec is happy again.

fixes #333590 (closed)

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Does this MR meet the acceptance criteria?

Conformity

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Security

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