Fix building against Ubuntu Noble, Debian testing
- debian/{control, rules}: define QT_SELECT instead of B-Ds on qt5-default
- src/, d/control: migrate to nlohmann_json 3.x & remove vendored copy
- include/, tests/: mark plugin descriptor as visible
- d/biometryd-bin.install: adjust to the new systemd unit location
- d/{control, rules}: adapt packaging to also be buildable on Debian