Add deprecation notice for Storybook 9 compatibility

Author: Guilherme da Silva Amorim

docs/docs/mocks/migration-to-storybook-9.md

@@ -0,0 +1,6 @@
+# Storybook Builder >> Migration from Storybook 8 to 9
+
+## General migration guide
+
+Storybook 9 introduced changes which consolidated external packages into the main Storybook packages. To avoid compatibility issues, `@web/mocks` won't be migrated and will keep support for Storybook 8, instead, we recommend using `@web/storybook-addons-mocks` which is compatible with Storybook 9.
+This is a drop-in replacement, so you can just change the import from `@web/mocks` to `@web/storybook-addons-mocks` and it should work without any issues while on version 9.

packages/mocks/README.md

@@ -1,5 +1,7 @@
 # @web/mocks
 
+## Deprecation notice: `@web/mocks` is compatible only up to Storybook 8. For Storybook 9, please use `@web/storybook-addons-mocks` instead.
+
 [`MSW`](https://mswjs.io/) integration layer for usage with [`@web/dev-server`](https://modern-web.dev/docs/dev-server/overview/), [`@web/test-runner`](https://modern-web.dev/docs/test-runner/overview/) and [`@web/dev-server-storybook`](https://modern-web.dev/docs/dev-server/plugins/storybook/#mainjs-and-previewjs).
 
 ## Defining mocks