Added Strawberry Shake runtime type change to migration guide (#8010)

Author: glen-84

website/src/docs/strawberryshake/v15/migrating/migrate-from-14-to-15.md

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+---
+title: Migrate Strawberry Shake from 14 to 15
+---
+
+This guide will walk you through the manual migration steps to update your Strawberry Shake GraphQL client to version 15.
+
+# Breaking changes
+
+Things that have been removed or had a change in behavior that may cause your code not to compile or lead to unexpected behavior at runtime if not addressed.
+
+## Runtime type changes
+
+The runtime type for the `Date` scalar is now `DateOnly` instead of `DateTime`.