chore: add deprecation note

Author: Julusian

DEVELOPER.md

@@ -147,6 +147,11 @@ However, one usage by AdlibActions for their userDataManifest remains as this is
 In R49, a replacement flow was added consisting of `validateConfig` and `applyConfig`.
 It is no longer recommended to use the old migrations flow for showstyle and studio blueprints.
 
+## Blueprint Migrations
+
+In R52, the replacement flow of `validateConfig` and `applyConfig` was extended to the system blueprint
+It is no longer recommended to use the old migrations flow for system blueprints.
+
 ### ExpectedMediaItems
 
 These are used for Media-manager which is no longer being developed.