As described in the post [here](https://itsfoss.com/key-is-stored-in-legacy-trusted-gpg/) you would need to export the key to its own file under the trusted.gpg.d directory.

So for example if the warning is for something like forticlient, first find the key using sudo apt-key gpg list and then taking the last eight characters and removing the space, export the key to its file (ie forticlient.gpg)

sudo apt-key export 5E54716D | sudo gpg --dearmour -o /etc/apt/trusted.gpg.d/forticlient.gpg