Dreprecate godep support (to be removed in 15.0)

deprecation asap and remove it in 15.0 as this would be a breaking change? gitlab-org/security-products/analyzers/license-finder!36 (comment 542645669)

implementation plan

• follow deprecation process in https://about.gitlab.com/handbook/marketing/blog/release-posts/#deprecations and at least:
  • Announce deprecation in release post and removal in 15.0
  • Add deprecation notice in documentation: https://docs.gitlab.com/ee/user/compliance/license_compliance/#supported-languages-and-package-managers