When I run Update manager in Linux Mint 21 x64 Cinnamon I get this error.
W: http://ppa.launchpad.net/lutris-team/lutris/ubuntu/dists/jammy/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
Post this over at the Mint forums - it is only a warning and Lutris will update. On the Mint forums there is a post for 20.1 showing how to solve the issue, but I don’t know if it works for 21.
I have a new install of Mint 21 and have the same issue across a number of ppa’s I normally use, but so far all seem to update (oibaf ppa is my reference for updates as they are released daily).