Add changeset for space migration (#10727)

### WHY are these changes introduced? Forgot to add a changeset when I merged [this](#10585) into `next`.
Shopify · Sep 27, 2023 · 179b481 · 179b481
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diff --git a/.changeset/lemon-bees-confess.md b/.changeset/lemon-bees-confess.md
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+---
+'@shopify/polaris': minor
+---
+
+Migrated `space` custom properties from v11 to v12