Support C.UTF-8 locale in the new builtin collation provider.

The builtin C.UTF-8 locale has similar semantics to the libc locale of
the same name. That is, code point sort order (fast, memcmp-based)
combined with Unicode semantics for character operations such as
pattern matching, regular expressions, and
LOWER()/INITCAP()/UPPER(). The character semantics are based on
Unicode simple case mappings.

The builtin provider's C.UTF-8 offers several important advantages
over libc:

 * faster sorting -- benefits from additional optimizations such as
   abbreviated keys and varstrfastcmp_c
 * faster case conversion, e.g. LOWER(), at least compared with some
   libc implementations
 * available on all platforms with identical semantics, and the
   semantics are stable, testable, and documentable within a given
   Postgres major version

Being based on memcmp, the builtin C.UTF-8 locale does not offer
natural language sort order. But it is an improvement for most use
cases that might otherwise use libc's "C.UTF-8" locale, as well as
many use cases that use libc's "C" locale.

Discussion: https://postgr.es/m/ff4c2f2f9c8fc7ca27c1c24ae37ecaeaeaff6b53.camel%40j-davis.com
Reviewed-by: Daniel Vérité, Peter Eisentraut, Jeremy Schneider

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202403191
+#define CATALOG_VERSION_NO	202403192
 
 #endif

src/include/catalog/pg_collation.dat

@@ -30,5 +30,8 @@
   descr => 'sorts using the Unicode Collation Algorithm with default settings',
   collname => 'unicode', collprovider => 'i', collencoding => '-1',
   colllocale => 'und' },
+{ oid => '811', descr => 'sorts by Unicode code point; Unicode and POSIX character semantics',
+  collname => 'pg_c_utf8', collprovider => 'b', collencoding => '6',
+  colllocale => 'C.UTF-8' },
 
 ]