doc: add release note for #31130

doc/release-notes-31130.md

@@ -0,0 +1,10 @@
+P2P and network changes
+-----------------------
+
+Support for UPnP was dropped. If you want to open a port automatically, consider using the `-natpmp`
+option instead, which uses PCP or NAT-PMP depending on router support.
+
+Updated settings
+------
+
+- Setting `-upnp` will now return an error. Consider using `-natpmp` instead.