Add updates for the GitLab Dedicated offering to the GitLab Tiers Handbook page.

Why is this change being made?

Pursuant to the limited availability of the GitLab dedicated offering, the GitLab tiers handbook page needs to be updated. A new Deployment Type column with the field values of Self Managed, Dedicated, and GitLab.com is added in the MR. Those names where chosen since that is how we should talk to customers about it. It makes sense to leave the delivery definition as is since the SaaS and Self-managed definitions are still valid. We need a new attribute and definition to describe tenancy. The Data Program needs these definitions in order to analyze and model the GitLab dedicated data.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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