Update IP allowlist information for Dedicated with 'lessons learned'

What does this MR do and why?

This MR updates the IP allowlist section of the Configure GitLab Dedicated docs to include some information we learned (see this Confidential work item - gitlab-com/gl-infra/gitlab-dedicated/team#3039). Specifically:

• Clarify what happens when an IP not on the allowlist attempts to access the instance
  • We used to say "the connection is refused". With this MR, that is clarified to "HTTP 403".
• Clarify a bit that the IP allowlist is not enabled by default
• Clarify that we add IPs to the allowlist when you send them in via a 🎫 ticket

MR acceptance checklist

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