Upsert::execute() return value is deprecated

Drupal\Core\Database\Query\Upsert::execute() returning a value is deprecated.

In Drupal 13, Drupal\Core\Database\Query\Upsert will implement QueryExecutionVoidInterface and its ::execute() method will be carrying a void return type.

For BC, during the deprecation period the method will keep returning an int value when called, unless in the Upsert options a 'return_affected_rows' => FALSE entry is added.