JSON_REPLACE() function executed with an error on Spider SE.
This patch fixes the problem, and it also fixes the MDEV-24541.

The problem is that Item_func_json_insert::func_name() returns
the wrong function name "json_update". 
The Spider SE reconstructs a query based on the return value
in some cases. Thus, if the return value is wrong, the Spider SE
may generate a wrong query.