input: Write a python function to convert all strings in a list to uppercase.
  assert to_uppercase(['Hello', 'World']) == ['HELLO', 'WORLD']
  assert to_uppercase(['python', 'java', 'c++']) == ['PYTHON', 'JAVA', 'C++']
  assert to_uppercase(['apple', 'banana', 'cherry']) == ['APPLE', 'BANANA', 'CHERRY']
output: def to_uppercase(strings):
      return [s.upper() for s in strings]