Add deprecation notice for migration to private repositories (#52)

Added prominent notice at the top of README files indicating these
  libraries are no longer open source and have moved to internal
  repositories. Includes links to the new private repository locations.

Author: xSeagullx

README.md

@@ -1,3 +1,11 @@
+> [!IMPORTANT]
+> **This repository is no longer open source**
+>
+> To better serve Wise business and customer needs, this library is no longer open source and will not receive further public updates. The codebase has moved to our internal systems.
+>
+> **For continued development and access, please refer to the internal repository:**
+> https://github.com/transferwise/tw-entrypoints-private
+
 # Tw-EntryPoints library
 
 ![Apache 2](https://img.shields.io/hexpm/l/plug.svg)