RaspbianOS does not create standard user pi with standard password raspberry any mor. That's good, During first boot you have to define the user and it's password now. If you setup your Raspberries headless you now have an issue.

Fortunately you can create a file userconf on the boot partition with one line of the following form:

username:encrypted- password

To create the encrypted password use following command:

echo 'mypassword' | openssl passwd -6 -stdin

Following oneliner will create file userconf on your boot partition:

echo "myuser:$(echo "mypassword" | openssl passwd -6 -stdin)" >  userconf

Referenz

An update to Raspberry Pi OS Bullseye - Simon Long (Raspberry Foundation)