namespace should be lowercased in kubernetes

What does this MR do?

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?

Autodevops fails for projects with uppercase in their name.

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?

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38692.