Drop go 1.12 support

Now both omnibus1 and CNG2 use Golang 1.13, we can drop support for Go 1.12. This would allow us to use 1.13 features if required. Further it allows us to only support Go versions which are suppported by Google.

This change removes the Go 1.12 jobs from the CI pipeline, and updates the documentation to reflect that one should use 1.13.9 or up from now on.