Update upgrade guide for required authentication for changing order a…

…nd order line metadata (#1433)

* Update upgrade guide for required authentication for changing order metadata

* Add info about fetching metadata

* Update the upgrade guide

docs/upgrade-guides/core/3-20-to-3-21.mdx

@@ -8,6 +8,12 @@ To follow the zero-downtime strategy when upgrading to 3.21, **It is recommended
 Otherwise, you will need to downtime your solution to ensure correct data migration.
 :::
 
+## Authentication required for changing `Order` and `OrderLine` metadata
+
+The [updateMetadata](/api-reference/miscellaneous/mutations/update-metadata.mdx) mutation for `Order` and `OrderLine` now requires the `MANAGE_ORDERS` permission.
+Once an order is created, customers cannot edit its metadata. However, merchants and apps with the necessary permissions can modify this data.
+Fetching metadata remains unchanged.
+
 ## Deprecated `manager.perform_mutation` method
 
 Note: This change only affects open-source Saleor users with custom plugins overriding the `perform_mutation` plugin method.