Update CustomersDot staging url in docs

What does this MR do?

CustomersDot staging URL was changed from customers.stg.gitlab.com to customers.staging.gitlab.com as part of the efforts in https://gitlab.com/gitlab-com/gl-infra/customersdot-ansible-poc/-/issues/54.

This MR updates the docs that mention how to connect your GitLab instance to CustomersDot staging.

Related issues

https://gitlab.com/gitlab-com/gl-infra/customersdot-ansible-poc/-/issues/54

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
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened