docs: add callout to only add new features to the main branch

README.md

 <!-- BEGIN_TF_DOCS -->
 # GCP Cloud Run Terraform Module
 
+> :bangbang: Following the release of version `9.0.0`, new features should
+> only be merged into the `master` branch. The legacy `v8` branch should only
+> receive critical bug fixes as needed.
+>
+> This is to encourage our products to migrate to the latest version of this
+> module, benefiting from its many improvements and standardising its usage
+> across our many infrastructure repositories.
+
 This module manages the deployment of containerised applications on Cloud Run.
 It includes the following functionality:
 

docs/templates/header.md

 # GCP Cloud Run Terraform Module
 
+> :bangbang: Following the release of version `9.0.0`, new features should
+> only be merged into the `master` branch. The legacy `v8` branch should only
+> receive critical bug fixes as needed.
+>
+> This is to encourage our products to migrate to the latest version of this
+> module, benefiting from its many improvements and standardising its usage
+> across our many infrastructure repositories.
+
 This module manages the deployment of containerised applications on Cloud Run.
 It includes the following functionality: