Many of these do depend on the Synapse master process (`matrix-synapse.service`),
so it makes sense to do it.

Furthermore, we're doing it so that one can stop the `matrix-synapse.service`
service and have systemd cascade this into stopping all the workers as well.

This is useful for easily stopping all of Synapse, so that Postgres
upgrades (`--tags=upgrade-postgres`) can happen cleanly.
Postgres upgrades currently stop `devture_postgres_systemd_services_to_stop_for_maintenance_list` which
includes Synapse, but stopping just the master process and leaving workers running is not safe enough and sometimes leads to errors like:

> ERROR:  insert or update on table "event_forward_extremities" violates foreign key constraint "event_forward_extremities_event_id"

With this dependency in place, stopping `matrix-synapse.service` will stop all Synapse processes.