Use C.UTF-8 as the default locale

The glibc 2.35-6 package ships with the C.UTF-8 locale included which means there is now a usable UTF-8 locale available by default.

en_US.UTF-8 will still be generated because PostgreSQL DBs are created with that locale:

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