use utf8mb4 instead of utf8 in TestCharset (#1228)
use utf8mb4 instead of utf8 in TestCharset (#1228)

From MySQL 8.0.24, `SELECT @@character_set_connection` reports utf8mb3 or utf8mb4 instead of utf8.
Because utf8 is currently an alias for utf8mb3, however at some point utf8 is expected to become a reference to utf8mb4.

> ref. https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-24.html#mysqld-8-0-24-bug
> Important Note: When a utf8mb3 collation was specified in a CREATE TABLE statement, SHOW CREATE TABLE, DEFAULT CHARSET,
> the values of system variables containing character set names,
> and the binary log all subsequently displayed the character set as utf8 which is becoming a synonym for utf8mb4.
> Now in such cases, utf8mb3 is shown instead, and CREATE TABLE raises the warning 'collation_name' is a collation of the deprecated character set UTF8MB3.
> Please consider using UTF8MB4 with an appropriate collation instead. (Bug #27225287, Bug #32085357, Bug #32122844)
>
> References: See also: Bug #30624990.

The document says that we should use utf8mb4 instead of utf8, so we should follow it.
methane/mysql,methane/mysql,go-sql-driver/mysql