GPL-2.0+ is a deprecated SPDX license identifier

Per https://www.drupal.org/node/2935137

Before:

$ composer validate
./composer.json is valid, but with a few warnings
See https://getcomposer.org/doc/04-schema.md for details on the schema
License "GPL-2.0+" is a deprecated SPDX license identifier, use "GPL-2.0+-only" or "GPL-2.0+-or-later" instead
require.drupal/lightning_api : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_core : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_layout : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_media : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_workflow : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning

After:

$ composer validate
./composer.json is valid, but with a few warnings
See https://getcomposer.org/doc/04-schema.md for details on the schema
require.drupal/lightning_api : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_core : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_layout : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_media : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning
require.drupal/lightning_workflow : exact version constraints (1.x-dev) should be avoided if the package follows semantic versioning