add ssl_mode given deprecation of require_ssl

[bobcallaway]

╷
│ Warning: Argument is deprecated
│ 
│   with module.sigstore.module.ctlog_shards["2022"].google_sql_database_instance.trillian,
│   on ../../../../../scaffolding/terraform/gcp/modules/mysql-shard/mysql.tf line 42, in resource "google_sql_database_instance" "trillian":
│   42:       require_ssl     = var.require_ssl
│ 
│ `require_ssl` will be fully deprecated in a future major release. For now, please use `ssl_mode` with a compatible `require_ssl` value instead.
│ 
│ (and 2 more similar warnings elsewhere)
╵