Update GitLab KAS installation doc

What does this MR do and why?

In omnibus-gitlab!6593 (merged) in the GitLab Omnibus repo, we are allowing the KAS address to be set through a new configuration: gitlab_kas_external_url

This MR updates the GitLab KAS installation to reflect the changes introduced above.

Issue: omnibus-gitlab#5784 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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