docs: add .server-changes note for the retired v3 dev CLI websocket

Author: nicktrn

.server-changes/retire-legacy-v3-dev-websocket.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: breaking
+---
+
+Removed support for the end-of-life v3 `trigger dev` CLI. Starting a dev session with an old v3 CLI now returns an upgrade message instead of connecting - upgrade to the v4 CLI to continue using `trigger dev`.