postgres: Allow specifying locale per-machine

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.

Replace en_US.UTF-8 with a variable where present and adapt the upgrade_pg.sh script to copy the locale from the old cluster.

playbooks/matrix.archlinux.org.yml

@@ -17,6 +17,7 @@
       postgres_work_mem: 64MB
       postgres_maintenance_work_mem: 256MB
       postgres_effective_cache_size: 4GB
+      postgres_locale: C
     - { role: postfix_null }
     - { role: matrix }
     - { role: fail2ban }