Setting MySQL 5.7 as the default tab on Google Cloud SQL

[sadhbh-a]

What does this PR do? What is the motivation?

The default tab on this this file is for MySQL 5.6, which is an older version of MySQL. It makes more sense for MySQL 5.7 to be the initial tab, as this is more useful for customers & is consistent with with the Postgres documentation with latest version first: https://docs.datadoghq.com/database_monitoring/setup_postgres/selfhosted/?tab=postgres15#grant-the-agent-access

Merge instructions

• [x] Please merge after reviewing

Additional notes