✨ Introduce trailing hyphens check

We saw .com production incident and a GitLab Dedicated incident related to 'trailing hyphens'. This check should help to make sure customers are not caught by surprise by this. (If folks upgrade to the latest stable release, of 17.6, 17.7 or 17.8, they should not be impacted. We saw this problem in 17.6.5, 17.7.4 and in 17.8.2.)

We stopped permitting folks to create new namespaces with trailing - characters some time ago. See gitlab-org/gitlab#356162 (closed).

The workaround we identified for folks who do not wish to rename the namespace was to use SSH for git requests.

📚 References

• Public Issue: gitlab-org/gitlab#519158 (closed)
• Production Issue: gitlab-com/gl-infra/production#19299 (closed)
• Dedicated Incident: gitlab-com/gl-infra/gitlab-dedicated/incident-management#709
• Dedicated Incident Review: gitlab-com/gl-infra/gitlab-dedicated/team#7840
• KB Article (US Gov): Git operations for projects in namespaces with trailing hyphens are broken