We have a few staff and users who prefer using a alias/mononym, so let's
make it optional.

Since our custom Keycloak theme does not use v3[1][2] of the account
console yet, it is not aware that the last name is optional, so
unfortunately the user cannot change "personal info" in the account
console without also specifying the last name. This only affects the
account console, the initial user configuration can be done without
specifying a last name.

[1] https://gitlab.archlinux.org/archlinux/keycloak-archlinux-theme/-/blob/238889fb36d512f0baa328fad00531ef53c14c1f/lib/src/main/resources/theme/archlinux/account/theme.properties#L1
[2] https://www.keycloak.org/2023/04/keycloak-2110-released#_experimental_account_console_version_3