locale: Use C.UTF-8 rather than en_US.UTF-8.

The `en_US.UTF-8` locale may not be configured or generated on all
installs; it also requires that the `locales` package be installed.
If users generate the `en_US.UTF-8` locale without adding it to the
permanent set of system locales, the generated `en_US.UTF-8` stops
working when the `locales` package is updated.

Switch to using `C.UTF-8` in all cases, which is guaranteed to be
installed.

Fixes #15819.

scripts/setup/create-db.sql

@@ -9,8 +9,8 @@ ALTER ROLE zulip SET search_path TO zulip,public;
 CREATE DATABASE zulip
     OWNER=zulip
     ENCODING=UTF8
-    LC_COLLATE='en_US.UTF-8'
-    LC_CTYPE='en_US.UTF-8'
+    LC_COLLATE='C.UTF-8'
+    LC_CTYPE='C.UTF-8'
     TEMPLATE=template0;
 \connect zulip
 CREATE SCHEMA zulip AUTHORIZATION zulip;