Upgrade to securego/gosec:2.0.0

What does this MR do?

This upgrade the securego/gosec base image to 2.0.0 which adds Go modules support for gosec.

As the default image now sets GO111MODULE to on, some logic is added for backwards compatibility for projects without Go modules support.

What are the relevant issue numbers?

• gitlab-org/gitlab-ee#12339

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer