Add unit testing for Go release candidates

What does this MR do?

Introduces CI image and unit test jobs against Go's latest release candidate.

Why was this MR needed?

We recently jumped from Go 1.13 to Go 1.17 and we're hoping to avoid such leaps in the future.

Although running integration tests might slow down the pipeline too much, running the unit tests might give us earlier confidence if we're continously testing against a future version as early as possible. In this case, it's using the rc tag, running 1.18beta1 at this point in time. Go 1.18 is due to be released in February.

What's the best way to test this MR?

N/A

What are the relevant issue numbers?

#28780 (closed)