The Steam Repository you are using is no good for your distribution. I am not sure if it is newer or older, but either way, it does not match your distro. Is your Kubuntu 22.04?
You also do not need a steam repository since it is in the Ubuntu Main Universe Repository.
You can remove the steam repository and apt key entirely, then in terminal, update your sources.
sudo rm /etc/apt/sources.list.d/steam.list
If Steam was added to your sources.list:
sudo nano /etc/apt/sources.list
and remove the line for Steam or hashtag (comment) it out.
It most likely is a steam.list in the sources.list.d directory, however.
For the key:
apt-key list
Look for Valve or Steam and grab its key I.D.
Copy it, then use it in
sudo apt-key del <KEY-I.D.>
For example, let's say the I.D. is ABCDEFG123456789:
sudo apt-key del ABCDEFG1F23456789
Once all is done and you have updated your sources:
sudo apt install --reinstall steam
EDIT: as per this post; if you are using a system in which apt-key is removed entirely, you can elevate to root:
sudo -i
Then launch your file manager
dolphin
(note to other readers: This instructs Dolphin, rather than nautilus because the user is on Kubuntu, not Zorin OS. Zorin OS users will enter nautilus for Core or Pro or thunar for Lite or Pro Lite).
Navigate to /etc/apt/trusted.gpg.d/ and search for the Valve or Steam pubkey there and right click to select the option to delete the file.