input: Write a function to sort a list of strings case insensitively.
  assert sort_strings_case_insensitive(['Banana', 'apple', 'Cherry', 'date']) == ['apple', 'banana', 'cherry', 'date']
  assert sort_strings_case_insensitive(['Hello', 'world', 'HELLO', 'WORLD']) == ['hello', 'hello', 'world', 'world']
  assert sort_strings_case_insensitive(['Aardvark', 'apple', 'banana', 'Cherry']) == ['aardvark', 'apple', 'banana', 'cherry']
output: def sort_strings_case_insensitive(strings):
      return sorted(strings, key=lambda s: s.lower())