Update KAS namespace on DEV

What does this MR do?

In gitlab-org/cluster-integration/gitlab-agent#300 (closed) we are coordinating the move for the KAS DEV mirror in order to suppor CNG build optimizations.

This MR updates the omnibus custom sources to point to the right mirror.

Related issues

Related to gitlab-org/cluster-integration/gitlab-agent#300 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes - n/a
• Documentation created/updated - n/a
• Tests added - n/a
• Integration tests added to GitLab QA - n/a
• Equivalent MR/issue for the GitLab Chart opened gitlab-org/build/CNG!1104 (merged)