Warn about this branch being deleted soon

Author: kennytv

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
+
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