matrix: use C locale for the synapse database

Synapse needs the database to be in C locale. Since v1.56.0, it refuses
to start when this is not the case, see [upgrade.md][1].

[1]: https://github.com/matrix-org/synapse/blob/v1.56.0/docs/upgrade.md#change-in-behaviour-for-postgresql-databases-with-unsafe-locale