Sanitize k8s default_namespace

What does this MR do?

Take over of ~"Community Contribution" https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14642

This is making namespace lowercase in kubernetes, otherwise namespaces cannot be created. This was partially addressed, but this covers a case that was missed when the default namespace is generated from the project group-name.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

~"auto devops" fails when project.path contains unallowed chars

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38692 (closed)