Follow-up to #42846.

Prior to this commit, `ActiveSupport::EncryptedFile` used the
`ActiveSupport::MessageEncryptor` default serializer.  #42846 changed
the default serializer to `JSON` when using `config.load_defaults 7.1`.
Thus, encrypted files that were written with the previous default
serializer (`Marshal`) could not be read by `Rails.application.encrypted`.
That included files which were written with `bin/rails encrypted:edit`
even when using the new default, because the `bin/rails encrypted:edit`
command does not run the initializer that applies the configuration to
`MessageEncryptor`:

  ```console
  $ bin/rails r 'puts ActiveSupport::MessageEncryptor.default_message_encryptor_serializer'
  json

  $ bin/rails encrypted:edit config/my_encrypted.yml.enc
  ...

  $ bin/rails encrypted:show config/my_encrypted.yml.enc
  foo: bar

  $ bin/rails r 'Rails.application.encrypted("config/my_encrypted.yml.enc").read'
  rails/activesupport/lib/active_support/message_encryptor.rb:241:in `rescue in _decrypt': ActiveSupport::MessageEncryptor::InvalidMessage (ActiveSupport::MessageEncryptor::InvalidMessage)
  ...
  ruby-3.2.0/lib/ruby/3.2.0/json/common.rb:216:in `parse': unexpected token at "foo: bar (JSON::ParserError)
  ```

This commit hard codes the serializer for `EncryptedFile` to `Marshal`.