Remove deprecated cache field in the docs

What does this MR do?

We recently removed the deprecated fields in the helm chart:

• gitlab-org/charts/gitlab-runner!393 (merged)
• gitlab-org/charts/gitlab-runner!397 (merged)

However, our documentation still reflects some of the deprecated fields.

For example, in setting up the cache, we still specify runners.cache in the documentation for setting up the authentication for the cache, but this is no longer supported in the helm chart.

Related issues

We have one instance where the customer is still using runners.cache after upgrading the Helm Chart to deploy GitLab Runner 16.0.1.

• https://gitlab.zendesk.com/agent/tickets/412260 (internal)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
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