websocket: add note about compat removal

this major release still needs to have an incompatible client, the next
one can drop setting a protocol client-side, and the one after that can
remove the protocol handling on the server side.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/PVE/APIServer/AnyEvent.pm

@@ -650,6 +650,8 @@ sub websocket_proxy {
 
 	    # todo: use stop_read/start_read if write buffer grows to much
 
+	    # FIXME: remove protocol in PVE/PMG 8.x
+	    #
 	    # for backwards, compatibility,  we have to reply with the websocket
 	    # subprotocol from the request
 	    my $res = "$proto 101 Switching Protocols\015\012" .