Update eligibility criteria text for premium pins (#11883)

Added eligibility note for US-based organizations.

## Summary of the problem
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any related issues to provide more details. -->

Costs for intl shipping are too high- decision has been made to be
US-only for the time being until we can find a solution.

## Describe your changes
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summary of the changes. -->

Added text that mentions it is for US-based organizations only.

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Co-authored-by: Gary Tou <[email protected]>

Author: alexdevz

app/views/events/perks/_premium_pins.html.erb

@@ -28,7 +28,7 @@
 
   <p>
     Ever wish you were just a little more dripped out in HCB swag?
-    We'll ship you some super cool custom HCB pins and keychains for free if you have 5 or more active teenagers!
+    We'll ship you some super cool custom HCB pins and keychains for free if you have 5 or more active teenagers! At the moment, only US-based teams are eligible for this.
   </p>
   <section class="card__banner card__darker secondary border-top italic">
     You currently have <%= active_teenagers_count %>/5 active teenagers in your organization!