WL#13342: Increase max value for max_prepared_stmt_count

Increase max value for max_prepared_stmt_count from 1 to 4 million (no change to
the default value of 16K)

https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_max_prepared_stmt_count

Motivation: We want to be able to test 40K connections using Sysbench with
prepared statements and up to 8 tables.

Sysbench with prepared statements and 8 tables gives 11 queries x 8 tables = 88
prepared statements per connection. Then 88 x 40K connections = 3520K prepared
statements

Memory usage: Estimated to 2 KB per prepared statement, so estimated max equals
to 4 million x 2KB = 8GB