Upgrade post - licensing cache issue

Why is this change being made?

We discovered a defect in the license caching logic in versions 15.2.5, 15.3.0 - 15.3.4 and 15.4.1. If a new license is added to the GitLab install, it may prevent premium features from working correctly.

This upgrade post describes the issue, the trigger and some workarounds.

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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