update the default collation of GBK from gbk_bin to gbk_chinese_ci (pingcap#20234) (pingcap#20238)

Author: ti-chi-bot

character-set-gbk.md

@@ -23,8 +23,9 @@ SHOW COLLATION WHERE CHARSET = 'gbk';
 | Collation      | Charset | Id   | Default | Compiled | Sortlen |
 +----------------+---------+------+---------+----------+---------+
 | gbk_bin        | gbk     |   87 |         | Yes      |       1 |
+| gbk_chinese_ci | gbk     |   28 | Yes     | Yes      |       1 |
 +----------------+---------+------+---------+----------+---------+
-1 rows in set (0.00 sec)
+2 rows in set (0.00 sec)
 ```
 
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