Move development to github.com/hifis.net

With the upcoming 5 users limit we decided to move the main project location to Github - https://github.com/hifis-net/ansible-role-gitlab.

As part of this several changes need to be made:

• Implement GitHub Actions pipeline
  • Draft: https://github.com/hifis-net/ansible-role-gitlab/pull/2
• Transfer open issues to GitHub
• Disable push mirroring
• Disable webhook for renovatebot
• Clearly reference the new project location on this page
• Adapt README that the project is archived
• Archive the repository to make it read-only.

\cc @Normo @tobiashuste