KNOWN_BUGS: connection migration doesn't work

Closes #7695

docs/KNOWN_BUGS

@@ -164,6 +164,7 @@ problems may have been fixed or changed somewhat since this was written!
  18.6 HTTP/3 multipart POST with quiche fails
  18.7 HTTP/3 quiche upload large file fails
  18.8 HTTP/3 doesn't support client certs
+ 18.9 connection migration doesn't work
 
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@@ -1163,3 +1164,7 @@ problems may have been fixed or changed somewhat since this was written!
  aka "mutual authentication".
 
  https://github.com/curl/curl/issues/7625
+
+18.9 connection migration doesn't work
+
+ https://github.com/curl/curl/issues/7695