License "GPL-2.0+" is a deprecated

Problem/Motivation

modules/conditional_fields$ 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-or-later" instead

Proposed resolution

Replace "GPL-2.0+" by "GPL-2.0-or-later"