Docs: Upgrade info for Helm Chart

What does this MR do and why?

When upgrading to GitLab Runner 15.0.0 using the Helm Chart project, the serviceAccount used by the Runner needs to have the get, list permissions for the serviceAccount resource.

More context can be found in this issue: gitlab-runner#29101 (comment 1014539304)

Screenshots or screen recordings

NA

How to set up and validate locally

NA

MR acceptance checklist

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