Update Go 1.14 to 1.14.12+

Go 1.14.12 has been released on 2020-11-12, and fixes two security issues (RCEs at build time):

• https://github.com/golang/go/issues/42560 (CVE-2020-28366)
• https://github.com/golang/go/issues/42557 (CVE-2020-28367)

We need to update these projects:

• CNG (from https://docs.gitlab.com/ee/development/go_guide/index.html#updating-go-version): https://gitlab.com/gitlab-org/charts/gitlab/-/issues/2655
• GitLab Omnibus Builder (according to this comment)
• GitLab Build Images [gitlab-build-images!382 (merged)]

/cc @gitlab-com/gl-security/appsec