Include JWT signing key in application secrets in docs

What does this MR do and why?

Describe in detail what your merge request does and why.

This aims to mention that as of 13.6, the JWT has a dedicated signing key for all installations.

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Current state: https://docs.gitlab.com/ee/development/application_secrets.html#secret-entries

Also... this still needs worked on. #221031 (closed)

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