Add check to krb5-user package

[kzz333]

Hello Maintainers,

1. From [1], the "heimdal-clients" package is conflicted with "krb5-user" package.

$ apt-cache show krb5-user | grep -e Package: -e Conflicts:
Package: krb5-user
Conflicts: heimdal-clients
Package: krb5-user
Conflicts: heimdal-clients

2. If "krb5-users" package had been removed, the end user may experience login failure intermittently behavior.

3. Hence, suggested a check if sssd-krb5-common had been installed, will warn if "krb5-user" package is not installed. The check will be triggered if Ubuntu 22 or older is used. The warning message does not suggest to force the package installation if no login fails intermittently issue.

if [ `lsb_release -rs | cut -f1 -d.` -le 22 ]; then
    if [ `dpkg -l | grep sssd-krb5-common | wc -l` -ne 0 ]; then
        if [ `dpkg -l | grep krb5-user | wc -l` -eq 0 ]; then
            echo '"sssd-krb5-common" is installed but "krb5-user" package is not installed. If the login fails intermittently, please install the "krb5-user" package. Please check with "apt-cache show krb5-user | grep -e Package: -e Conflicts:"'
        fi
    fi
fi

Thank you.

Best Regards,
Koo

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810681