- `install-service` no longer forces `one-by-one` restart mode

- the coturn priority condition is flipped: only `one-by-one` mode
  needs the delayed priority (1500); all other modes (including
  the new `all-at-once` default) use the normal priority (900)

Ref:

- d42cd92045
- f3e658cca3/docs/restart-mode-comparison.md
- 36445fb419
- 750cb7e29e