Add a script to delete projects directly under the sandbox group

What does this MR do?

Implements the second task of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/722:

• Write a script to delete the projects directly under the sandbox group similar to the existing delete_subgroups script.

To run the task on your local:

GITLAB_QA_ACCESS_TOKEN=<add-token-here> GITLAB_ADDRESS="http://localhost:3000" rake delete_projects

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content