Deprecate download-secure-files

When Mobile DevOps CLI Integration Support Request (#44 - closed) is complete, the next step will be to deprecate the download-secure-files tool in favor of the supported glab CLI implementation. Below is a list of tasks that should be completed to deprecate the tool.

• Add a note to the README and archive the project repository https://gitlab.com/gitlab-org/incubation-engineering/mobile-devops/download-secure-files
• Update the usage instructions in the docs 1 gitlab-org/gitlab!209009 (merged)
• Update instructions in blog posts 1, 2 gitlab-com/marketing/digital-experience/about-gitlab-com!2515 (merged)
• Update instructions in tutorials 1 gitlab-org/gitlab!209009 (merged)
• Respond to / close open issues that can be closed with the availability of this feature in glab