licensing: use GPL-3.0 or later

Signed-off-by: Santiago M. Mola <santi@mola.io>

Author: smola

engineering/project-template/README.md

@@ -89,7 +89,7 @@ Apache License Version 2.0, see [LICENSE.md](LICENSE.md).
 or
 ```
 
-GPL v3.0, see [LICENSE.md](LICENSE.md).
+GPL v3.0 or later, see [LICENSE.md](LICENSE.md).
 
 ```
 or