Patch 1 of GB18030-2022 support

Hi John and all,

Per John's suggestion from the discussion
https://www.postgresql.org/message-id/CANWCAZY%3D6qq3obTNyUXq3gVa2an8mrC4c4SoDSi44NB9T0osOw%40mail.gmail.com,
I am splitting the original patch of upgrading to GB18030-2022 into
multiple patches.

In the first patch, I only replaced gb-18030-2000.xml with
gb-18030-2000.ucm. I picked up a version of the ucm file that matches to
the current xml file so that this patch focuses on a simple goal of
replacing xml with ucm. The generated map files have no change:

```

chaol(at)ChaodeMacBook-Air Unicode % perl -I. UCS_to_GB18030.pl
- Writing UTF8=>GB18030 conversion table: utf8_to_gb18030.map
- Writing GB18030=>UTF8 conversion table: gb18030_to_utf8.map
chaol(at)ChaodeMacBook-Air Unicode % git diff
chaol(at)ChaodeMacBook-Air Unicode %

```

So this patch should have no any impact to build-out binaries of PostgreSQL.

Patch 2 will update gb-18030-2000.ucm with the latest version, and patch
3 will upgrade ucm to the 2022 version.

Best regards,

Chao Li (Evan)
--------------
HighGo Software Co., Ltd.
https://www.highgo.com/