common: Replace deprecated systemd-swap[1] with zram-generator

zram-generator defaults to 50% of the ram where systemd-swap defaults to
25% of the ram.

[1] https://lists.archlinux.org/pipermail/arch-dev-public/2021-May/030429.html

---

This has already been deployed on dashboards.archlinux.org manually to test the configuration and figure out how to activate it (daemon-reload + restart swap.target).