Add v12-react-avatar-component migration (#10874)

In order to run the `v12-react-avatar-component` migration as part of
our beta release it needs to be merged and released on `main` instead of
the `next` branch.

Ports over `v12-react-avatar-component` migration from [[Avatar] Remove
customer prop and experiemntal values

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Co-authored-by: Lo Kim <lo.kim@shopify.com>
Co-authored-by: Lo Kim <26749317+laurkim@users.noreply.github.com>

Author: 2 people

.changeset/spotty-garlics-kick.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris-migrator': minor
+---
+
+Ported `v12-react-avatar-component` migration from [Avatar] Remove customer prop and experimental values [#10283](https://github.com/Shopify/polaris/pull/10283) to main branch