Fix typo on shared-secrets doc and make deprecations explicit

What does this MR do?

Fixes typos: Those references are not Gitaly ones, but shared-secrets related. This as very likely a copy/paste error.

More so, these shared-secrets.image.* have been deprecated in favor of global.kubectl.image.* since 2019: !864 (diffs).

It's easier to acknowledge their deprecations when learning about them from the docs, than testing the configs, and only learning about the deprecations when the Helm install command complains to you.

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