Add deprecation note to the README

Point users to the `lstk` CLI with links to the getting-started and
migration guides, in line with the notice design proposed in DEVX-1126.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: joe4dev

README.md

@@ -1,6 +1,14 @@
 LocalStack CLI
 =======================
 
+> [!WARNING]
+> **The `localstack` CLI is deprecated. Use the `lstk` CLI instead.**
+>
+> The `localstack` CLI no longer receives updates. Existing commands keep working for now, but new features and fixes only land in `lstk`.
+>
+> - Get started with `lstk`, the new LocalStack CLI: [docs.localstack.cloud/.../lstk](https://docs.localstack.cloud/aws/developer-tools/running-localstack/lstk/)
+> - Migrate local scripts and CI workflows: [docs.localstack.cloud/.../lstk/migration](https://docs.localstack.cloud/aws/developer-tools/running-localstack/lstk/migration/)
+
 This repository contains building instructions for binary builds of the LocalStack CLI.
 It does not contain the actual source for the CLI, since the LocalStack CLI is basically just the Python package `localstack` (published on PyPi) with it's install dependencies (and without any extras).
 This is why this repository just contains the build config and pipeline that packages the LocalStack CLI python package into a standalone binary using PyInstaller.