mailserver: bump stateVersion to 5 (Sieve scripts in Dovecot home)
simple-nixos-mailserver now stores per-user Sieve scripts in each user's
Dovecot home (~/sieve, active link ~/.dovecot.sieve) instead of the old
global /var/sieve, and asserts stateVersion >= 5 when enableManageSieve is
set. Existing scripts were migrated with migration #5 on neoprism.
Assisted-by: Claude Code (claude-opus-4-8[1m])