doc: Add release notes for 22570 (ignore banlist.dat)

doc/release-notes.md

@@ -66,6 +66,16 @@ New RPCs
 Build System
 ------------
 
+Files
+-----
+
+* On startup, the list of banned hosts and networks (via `setban` RPC) in
+  `banlist.dat` is ignored and only `banlist.json` is considered. Bitcoin Core
+  version 22.x is the only version that can read `banlist.dat` and also write
+  it to `banlist.json`. If `banlist.json` already exists, version 22.x will not
+  try to translate the `banlist.dat` into json. After an upgrade, `listbanned`
+  can be used to double check the parsed entries. (#22570)
+
 New settings
 ------------