Warn about this branch being deleted soon

README.md

@@ -1,3 +1,8 @@
+> [!CAUTION]
+> **This branch will be deleted soon, please replace usage of it with the new `main` branch**.
+>
+> If you need history from before 1.21.4, please use the archive repo: https://github.com/PaperMC/Paper-archive
+
 Paper [![Paper Build Status](https://img.shields.io/github/actions/workflow/status/PaperMC/Paper/build.yml?branch=master)](https://github.com/PaperMC/Paper/actions)
 [![Discord](https://img.shields.io/discord/289587909051416579.svg?label=&logo=discord&logoColor=ffffff&color=7389D8&labelColor=6A7EC2)](https://discord.gg/papermc)
 [![GitHub Sponsors](https://img.shields.io/github/sponsors/papermc?label=GitHub%20Sponsors)](https://github.com/sponsors/PaperMC)