Gitaly StatefulSet name change in docs

What does this MR do?

Customers who have deployed Gitaly cluster using the helm chart will have issues at 13.8.

It's in the release notes, but I didn't look there. I looked in issues and in the docs.

▶ Breaking news, we suspect the customer has done this, and it doesn't work. Probably still worth getting this in the docs, but more steps may be needed.

Related issues

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