Bump minimum required Go version for Workhorse to 1.15

What does this MR do?

We're currently in the process of moving to Go 1.16 across all distributions, with a minimum required Go version of 1.15. Workhorse is still using Go 1.13 as the minimum required Go version, which doesn't make a lot of sense when other components already require a newer version anyway.

Bump the minimum required version of Go to 1.15 and move over the default image in CI to 1.16.

Does this MR meet the acceptance criteria?

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