input: Write a function to generate prime numbers up to a given number.
  assert generate_prime_numbers(10) == [2, 3, 5, 7]
  assert generate_prime_numbers(20) == [2, 3, 5, 7, 11, 13, 17, 19]
  assert generate_prime_numbers(1) == []
output: def generate_prime_numbers(n):
      if n < 2:
          return []
      
      primes = []
      for possible_prime in range(2, n + 1):
          is_prime = True
          for num in range(2, int(possible_prime ** 0.5) + 1):
              if possible_prime % num == 0:
                  is_prime = False
                  break
          if is_prime:
              primes.append(possible_prime)
      
      return primes