Replace gopkg.in/alecthomas/kingpin.v2 by github.com/alecthomas/kingpin/v2

[gravufo]

Description of your changes

Fixes #388

I have:

• [x] Read and followed Crossplane's [contribution process].
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

None, this should have no impact, it is essentially the same module reference.