lib/json{,_fwd}.hpp: upgrade to 3.9.1
Such an upgraded library is needed to compile on some non-standard systems.
A GitLab upgrade is scheduled for Monday April 20th. The service will be unavailable from 9 am to 12. We'll keep you posted on the progress of the upgrade on the Mattermost channel: https://mattermost.inria.fr/devel/channels/gitlab. We recommend that you do not work on the platform until an announcement indicates that maintenance is complete.
Such an upgraded library is needed to compile on some non-standard systems.