Write a migration guide.

Author: andriyDev

release-content/migration-guides/validate_parent_has_component_is_now_a_plugin.md

@@ -0,0 +1,8 @@
+---
+title: `validate_parent_has_component` is superseded by `ValidateParentHasComponentPlugin`
+pull_requests: []
+---
+
+The `validate_parent_has_component` insert hook has been replaced by a plugin:
+`ValidateParentHasComponentPlugin`. This uses an observer, a resource, and a system to achieve a
+more robust (and less spurious) warning for invalid configuration of entities.