Update vtgate advertised mysql version

[harshit-gangal]

Currently, VTGate advertises 5.7.9 as the MySQL version to the clients.
The MySQL 5.7 version is going EOL in October 2023.
Mysql 8.0 was launched in April 2018 and it has been 4+ years. Any new installations are happening on the 8.0 versions. There are some MySQL clients/frameworks that rely on VTGate's advertised MySQL version.

In Vitess CI, we have also upgraded our environments to use the latest version of MySQL (8.0.31 released in October 2022).

The plan is to upgrade the advertised version to 8.0.31 and old installations can change the default by providing the mysql_server_version flag on the VTGate.