Updating Zendesk retention period to align with guidance to Support Operations

Why is this change being made?

Support Team and Privacy discussed setting fixed deletion schedules for free and paid tickets/users in Zendesk, per this Issue. After researching suggested retention timeframes and checking against our contractual obligations, Support and Privacy agreed on the following schedule:

• 3-years from the point of termination of the paid contractual relationship for tickets/users in Zendesk
• 3-years from the point of ticket creation for free users

Thus, this updates the Zendesk ticket retention minimum to 3-years.

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
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  • 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
• 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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