make ipv4_enabled default to true

[modular-magician]

Fixes hashicorp/terraform-provider-google#6012.

Adding a default value will only affect people who had already provisioned instances that did not set a value for that field. If they're a second gen user, the server-side default is true, so their state would already have this value set as true. First gen instances have been decommissioned, so there should be ~0 terraform users using them.

Release Note Template for Downstream PRs (will be copied)

* sql: fixed error that occurred on `google_sql_database_instance` when `settings.ip_configuration` was set but `ipv4_enabled` was not set to true and `private_network` was not configured, by defaulting `ipv4_enabled` to true.

Derived from GoogleCloudPlatform/magic-modules#3330