Change mixnode, gateway, network and nym-api binary licenses to GPLv3 (#4173)

* Change mixnode, gateway, network and nym-api binary licenses to GPLv3

* Add license to cargo files

* Revert model license to match crate

* Add license to nym-node

* Revert model license to match crate

* Fix formatting

---------

Co-authored-by: Mark Sinclair <mmsinclair@users.noreply.github.com>

mixnode/src/node/node_description.rs

@@ -1,3 +1,6 @@
+// Copyright 2023 - Nym Technologies SA <contact@nymtech.net>
+// SPDX-License-Identifier: GPL-3.0-only
+
 use serde::Deserialize;
 use serde::Serialize;
 use std::path::Path;