chore: update README with deprecation notice

README.md

@@ -1,4 +1,7 @@
-# Flipt Validate Action
+# Flipt Validate Action (Deprecated)
+
+> [!NOTE]
+> This action is deprecated and will not be maintained going forward. Please use the [Flipt Setup Action](https://github.com/marketplace/actions/flipt-setup-action) instead which can be used to validate your feature flag files as perform any other operations provided by the [Flipt CLI](https://www.flipt.io/docs/cli/overview).
 
 ![GitHub Release](https://img.shields.io/github/v/release/flipt-io/validate-action)