Move sysctl settings (esp. net.ipv4.ip_forward=1!) to local.conf.

Move sysctl settings (esp. net.ipv4.ip_forward=1!) to local.conf.

Debian 12's Systemd installed /etc/sysctl.d/99-sysctl.conf, a symlink
to /etc/sysctl.conf, so systemd-sysctl has not included sysctl.conf
for a while.  Debian 13 has no /etc/sysctl.d/99-* and no
/etc/sysctl.conf, so Ansible's sysctl builtin was less helpful than
a simple copy to /etc/sysctl.d/local.conf.