If in doubt set the local ip-address of your DHCP server (usually your router’s ip address) here.
A list of public DNS servers can be found here or a more extensive list is here (amongst many other lists).
As for Debian 12 (bookworm):
“The new systemd-resolved package will not be installed automatically on upgrades. You need to install the new package manually. Note that until it has been installed, DNS resolution might no longer work since the service will not be present on the system.” (More about the latter here)