add forgotten migration guide (#19800)

# Objective

- Forgot to add a migration guide on #19789

## Solution

- Add one

Author: atlv24

release-content/migration-guides/compressed-image-saver.md

@@ -0,0 +1,6 @@
+---
+title: Compressed image saver feature
+pull_requests: [19789]
+---
+
+The compressed image saver has been gated behind its own dedicated feature flag now. If you were using it, you need to enable the "compressed_image_saver" feature.