Export and upgrade version of Go

What does this MR do?

• export version of Go as GO_VERSION
• use GO_VERSION as a Docker build argument
• upgrade to Go 1.14.6

The upgrade is debatable but some projects using multi-stage Dockerfiles already use Go 1.14.

Analyzer projects that appear to be incompatible with Go 1.14 can revert to Go 1.13.6 by setting GO_VERSION in their CI configuration file, making the compatibility issue visible in the codebase.

What are the relevant issue numbers?

Related to gitlab-org/gitlab#9736 (closed)

Could be leveraged to upgrade to Go 1.14, as described in gitlab-org/gitlab#223270 (closed) and gitlab-org/gitlab#223268 (closed)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
  • tested with single-stage Dockerfile: https://gitlab.com/gitlab-org/security-products/analyzers/bundler-audit
    • go build
  • tested with multi-stage Dockerfile: https://gitlab.com/gitlab-org/security-products/analyzers/gosec
    • go build
    • docker build (1st stage)
    • docker build (2nd stage) Same layer.