[PLAY-1527] Hosting the right phone number input flags png file (#3641)

**What does this PR do?** 
[Runway Story](https://runway.powerhrg.com/backlog_items/PLAY-1527)


**How to test?** Steps to confirm the desired behavior:
1. Go to the Phone Number Input kit doc page.


#### Checklist:
- [x] **LABELS** Add a label: `enhancement`, `bug`, `improvement`, `new
kit`, `deprecated`, or `breaking`. See [Changelog &
Labels](https://github.com/powerhome/playbook/wiki/Changelog-&-Labels)
for details.
- [x] **DEPLOY** I have added the `milano` label to show I'm ready for a
review.
- [ ] **TESTS** I have added test coverage to my code.

playbook/app/pb_kits/playbook/pb_phone_number_input/_phone_number_input.scss

@@ -60,6 +60,7 @@ $flag-min-resolution: 192dpi;
   }
 
   .iti__flag {
+    background-image: url("https://unpkg.com/[email protected]/dist/assets/flags.png");
     border-radius: 1px;
   }
 
@@ -196,4 +197,10 @@ $flag-min-resolution: 192dpi;
       }
     }
   }
+
+  @media (-webkit-min-device-pixel-ratio: 2), (min-resolution: $flag-min-resolution) {
+    .iti__flag {
+      background-image: url("https://unpkg.com/[email protected]/dist/assets/flags.png");
+    }
+  }
 }