Add a migration guide for the new use_asset_processor_override field.

Author: andriyDev

release-content/migration-guides/asset_plugin_processed_override.md

@@ -0,0 +1,8 @@
+---
+title: `AssetPlugin` now has a `use_asset_processor_override` field.
+pull_requests: [21409]
+---
+
+The `AssetPlugin` now has a `use_asset_processor_override` field. If you were previously setting all
+`AssetPlugin` fields, you should either use struct update syntax `..Default::default()`, or set the
+new `use_asset_processor_override` to `None`.