readme: add deprecation banner (#6838)

README.md

@@ -1,3 +1,14 @@
+> **Deprecation notice**: Grafana Agent has been deprecated and is now in
+> Long-Term Support mode. We recommend migrating to the new Grafana Alloy
+> collector, which is built on the foundation of Grafana Agent Flow.
+>
+> For more information, read our blog posts about Alloy and how to easily
+> migrate from Agent to Alloy:
+>
+> * [Alloy announcement blog post](https://grafana.com/blog/2024/04/09/grafana-alloy-opentelemetry-collector-with-prometheus-pipelines/)
+> * [Alloy FAQ](https://grafana.com/blog/2024/04/09/grafana-agent-to-grafana-alloy-opentelemetry-collector-faq/)
+> * [Migrate to Alloy](https://grafana.com/docs/alloy/latest/tasks/migrate/)
+
 <p align="center"><img src="docs/sources/assets/logo_and_name.png" alt="Grafana Agent logo"></p>
 
 Grafana Agent is an OpenTelemetry Collector distribution with configuration