WIP: Delete CONTRIBUTING.md

Related issues

Developer checklist

• Link to the developer security workflow issue on dev.gitlab.org
• MR targets master, or X-Y-stable for backports
• Milestone is set for the version this MR applies to
• Title of this MR is the same as for all backports
• A CHANGELOG entry is added without a merge_request value, with type set to security
• Add a link to this MR in the links section of related issue
• Set up an EE MR (always required for CE merge requests): EE_MR_LINK_HERE
• Assign to a reviewer (that is not a release manager)

Reviewer checklist

• Correct milestone is applied and the title is matching across all backports
• Assigned to @gitlab-release-tools-bot with passing CI pipelines