Carbon 1 is deprecated - migrate to Carbon 2
To find the culprit, you could run a composer show --tree to see which plugin is using the dependency.

So far, I found two plugins using it :


rias/craft-contact-form-extensions (1.1.6)
solspace/craft3-calendar (2.0.20)


If you're using Solspace calendar, you should consider to upgrade to v3.
If you're using Rias Contact form extension, you can try to add the updated dependency to your composer.json file by running a composer require nesbot/carbon. It will fix the warning until the plugin is updated.