composer.json License is Invalid/Deprecated

Just started getting error notices from Packagist every time I push a commit to any extension (my own and official ones) and pretty sure any one connected to phpBB's packagist account should start getting some too.

Packagist can no longer update from our repos because our composer.json is now invalid.

Just run

composer validate

to see the following error:

Invalid package information: 

License "GPL-2.0" is a deprecated SPDX license identifier, use "GPL-2.0-only" or "GPL-2.0-or-later" instead

Valid license names listed here: https://spdx.org/licenses/

So apparently we need to change from:

"license": "GPL-2.0"

to either

"license": "GPL-2.0-only"

or

"license": "GPL-2.0-or-later"