pgsql: Don't allow creation of database with ICU locale with unsupporte

Don't allow creation of database with ICU locale with unsupported encoding

Check in CREATE DATABASE and initdb that the selected encoding is
supported by ICU.  Before, they would pass but users would later get
an error from the server when they tried to use the database.

Also document that initdb sets the encoding to UTF8 by default if the
ICU locale provider is chosen.

Author: Marina Polyakova <m.polyakova@postgrespro.ru>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/6dd6db0984d86a51b7255ba79f111971@postgrespro.ru

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/148f66d59e2c2fa8bc79918973874e2803892664

Modified Files
--------------
doc/src/sgml/ref/initdb.sgml      |  5 +++--
src/backend/commands/dbcommands.c |  9 +++++++--
src/bin/initdb/initdb.c           | 29 +++++++++++++++++++++++++++++
src/bin/initdb/t/001_initdb.pl    |  9 +++++++++
src/bin/scripts/t/020_createdb.pl |  9 +++++++++
5 files changed, 57 insertions(+), 4 deletions(-)