old key message inmint 21

Hello, in Mint Cinnamon 21 after installation:
$ sudo add-apt-repository ppa:mixxx/nightlies
$ sudo apt update
$ sudo apt install mixxx

After the command: sudo apt update

This Message comes:
W: http://ppa.launchpad.net/mixxx/nightlies/ubuntu/dists/jammy/InRelease: Schlüssel ist im veralteten Schlüsselbund trusted.gpg gespeichert (/etc/apt/trusted.gpg), siehe den Abschnitt MISSBILLIGUNG in apt-key(8) für Details.

But Mixxx works

Note: At the install instructions is still standing: Ubuntu 20.04 "Focal Fossa" or later
I mean that that for develompment packages the hint to to 22.04.1 would be right.