input: Write a python function to create a list of Employee objects and filter out those with a salary greater than a given amount.
  assert len(filter_employees([Employee("John", 1, 50000, "HR"), Employee("Jane", 2, 60000, "Finance"), Employee("Doe", 3, 45000, "IT")], 55000)) == 1
  assert len(filter_employees([Employee("Alice", 4, 70000, "IT"), Employee("Bob", 5, 80000, "HR")], 75000)) == 1
  assert len(filter_employees([Employee("Charlie", 6, 40000, "Finance"), Employee("Dave", 7, 35000, "IT")], 30000)) == 2
output: class Employee:
      def __init__(self, name, id, salary, department):
          self.name = name
          self.id = id
          self.salary = salary
          self.department = department
  
  def filter_employees(employee_list, salary_threshold):
      return [employee for employee in employee_list if employee.salary <= salary_threshold]