reference: update supported character sets (#2174)
reference: update supported character sets (#2174)

* reference: update supported charater sets

* reference: fix a sql statement

* reference: fix code block format

Co-authored-by: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>

pingcap/docs-cn,pingcap/docs-cn