Use `declarative-policy` as project for Reviewer Roulette

Description

This MR upgrade gitlab-dangerfiles to 3.8 and uses declarative-policy as project name again to lookup reviewers/maintainers for danger' Reviewer Roulette because with #27 (closed) we do have some folks now 🎉

See https://about.gitlab.com/handbook/engineering/projects/#declarative-policy

Related Issues

Refs #27 (closed)

Suggested version bump

• Major (backwards incompatible changes)
• Minor (backwards compatible changes)
• Patch (API compatible changes)

Checklist

• [-] Tests have been added or updated to cover any changes in behavior
• [-] This does not change the API to consume this library, or a suggested version bump has been provided
• [-] The CHANGELOG.md has been updated
• No new runtime dependencies have been introduced