Change PostgreSQL cluster locale to C.UTF-8

After e74066e3, the only remaining thing that uses the en_US.UTF-8 locale is PostgreSQL:

• roles/postgres/files/upgrade_pg.sh
• roles/postgres/tasks/main.yml
• roles/postgres/templates/postgresql.conf.j2

Changing the files is not enough, the PostgreSQL clusters need to be migrated to the C.UTF-8 locale by dumping and recreating them with initdb --locale=C.UTF-8 --encoding=UTF8 ....