Make sure we generate keys that can be used with the cipher

We use aes-256-cbc cipher by default and it only accepts keys with 32
bytes at max.

Closes #27576.

[ci skip]

activesupport/lib/active_support/message_encryptor.rb

@@ -14,10 +14,10 @@ module ActiveSupport
   # where you don't want users to be able to determine the value of the payload.
   #
   #   salt  = SecureRandom.random_bytes(64)
-  #   key   = ActiveSupport::KeyGenerator.new('password').generate_key(salt) # => "\x89\xE0\x156\xAC..."
-  #   crypt = ActiveSupport::MessageEncryptor.new(key)                       # => #<ActiveSupport::MessageEncryptor ...>
-  #   encrypted_data = crypt.encrypt_and_sign('my secret data')              # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
-  #   crypt.decrypt_and_verify(encrypted_data)                               # => "my secret data"
+  #   key   = ActiveSupport::KeyGenerator.new('password').generate_key(salt, 32) # => "\x89\xE0\x156\xAC..."
+  #   crypt = ActiveSupport::MessageEncryptor.new(key)                           # => #<ActiveSupport::MessageEncryptor ...>
+  #   encrypted_data = crypt.encrypt_and_sign('my secret data')                  # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
+  #   crypt.decrypt_and_verify(encrypted_data)                                   # => "my secret data"
   class MessageEncryptor
     DEFAULT_CIPHER = "aes-256-cbc"