Change "GPL-3.0+" to "GPL-3.0-or-later"

The "GPL-3.0+"-SPDX-license-identifier has been deprecated,
and was replaced with "GPL-3.0-or-later".

More information about that, is available in the
"Deprecated License Identifiers"-part of
https://spdx.org/licenses/ .

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/642>

src/server-main.c

@@ -2,7 +2,7 @@
  * Copyright (C) 2010-2011 Daiki Ueno <ueno@unixuser.org>
  * Copyright (C) 2010-2011 Red Hat, Inc.
  * Copyright (C) 2018-2019 Purism SPC
- * SPDX-License-Identifier: GPL-3.0+
+ * SPDX-License-Identifier: GPL-3.0-or-later
  * Author: Guido Günther <agx@sigxcpu.org>
  *
  * This program is free software: you can redistribute it and/or modify