Change minimum UID/GID for LDAP user to 2000

With this change local user accounts now use the UID/GID range 1000-1999 instead of 500-999 whereas LDAP user accounts use 2000-59999 instead of 1000-59999. This is to reserve UID/GID 0-999 for system users which is the default in Debian and not conforming to it is increasingly problematic as packages are beginning to use systemd-sysusers for creating system user accounts which does not obey /etc/addusers.conf or /etc/login.defs by default.

The first user account created during installation now has UID/GID 2000 instead of 1000.

Configure gosa and adjust ldap-createuser-krb5 accordingly.

Closes: #1003192.

This change should only got into unstable and must be merged together with changes to debian-edu-install!4 (merged) and debian-edu/upstream/libpam-mklocaluser!4 (merged).