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/actors/mod.rs

@@ -1,5 +1,5 @@
 /* Copyright (C) 2022 Purism SPC
- * SPDX-License-Identifier: GPL-3.0+
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /*! Actors are parts of Squeekboard containing state independent from the main application state.

src/actors/popover.rs

@@ -1,5 +1,5 @@
 /* Copyright (C) 2022 Purism SPC
- * SPDX-License-Identifier: GPL-3.0+
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /*! The popover is opened directly by the GTK surface,