require golang 1.18 for KD build

[joel-bluedata]

This will put some CVEs in our rearview mirror.

JFrog scan of 0.10.1 image: 4 critical, 15 high
JFrog scan of master branch build using golang 1.18.5: 1 critical, 8 high

The remaining critical is CVE-2022-1996 from emicklei/go-restful ... we could try to force an update on that package or we might be required to finally move to a more recent version of operator SDK. I'm not super exercised about that one though because as described in emicklei/go-restful#489 it does not apply to KD.