Docs: Omnibus requires perl for private TLS roots

What does this MR do?

While working a ticket and troubleshooting a customer's (internal links) use of TLS for Gitaly we identified that gitlab-ctl reconfigure was not integrating private CA root certificates placed in /etc/gitlab/trusted-certs on their Omnibus GitLab installation.

The Omnibus docs do call this out within the documentation about adding certificates, but it's not covered off as a pre-requisite.

The customer, on the call, suggested that this should be in the docs. Their server build was defined with a tool like Puppet or Chef, and this was missed as a base requirement as it's not in the base requirements section of our docs.

As Omnibus links back to our core docs for installation requirements, I've added it there.

Related issues

• omnibus-gitlab#4288 (closed)
• omnibus-gitlab#2275 (closed)

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