input: Write a python function to filter customers who are at least 21 years old.
  assert filter_customers(customers) == [{'name': 'John', 'age': 21}, {'name': 'Minh', 'age': 24}]
  assert filter_customers([]) == []
  assert filter_customers([{'name': 'Alice', 'age': 17}]) == []
output: def filter_customers(customers):
      return [customer for customer in customers if customer['age'] >= 21]