chore: Add Rails 7.1

[MrSerth]

This PR adds support for Rails 7.1.

⚠️ While the test suite passes, the changes made here are not enough.

Consider the following example:

Failing example

# frozen_string_literal: true

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  git_source(:github) {|repo| "https://github.com/#{repo}.git" }

  gem 'mnemosyne-ruby'
  gem 'rails', '~> 7.1.0'
end

require 'rack/test'
require 'action_controller/railtie'

class TestApp < Rails::Application
  config.root = __dir__
  config.hosts << 'example.org'
  config.session_store :cookie_store, key: 'cookie_store_key'
  config.secret_key_base = 'secret_key_base'

  mnemosyne_config = ::Mnemosyne::Configuration.new('server' => 'amqp://localhost', 'application' => 'TestApp',
    'hostname' => 'example.org')
  config.middleware.insert 0, ::Mnemosyne::Middleware::Rack
  ::Mnemosyne::Instrumenter.start!(mnemosyne_config)

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  routes.draw do
    get '/' => 'test#index'
  end
end

class TestController < ActionController::Base
  include Rails.application.routes.url_helpers

  def index
    render plain: 'Home'
  end
end

require 'minitest/autorun'

class BugTest < Minitest::Test
  include Rack::Test::Methods

  def test_returns_success
    get '/'
    assert last_response.ok?
  end

  def test_returns_not_found
    get '/not_found'
    assert last_response.not_found?
  end

  private

  def app
    Rails.application
  end
end

Executing this script (just by invoking it with ruby <filename>) will fail for Rails 7.1. If tested against Rails 7.0 (changing gem 'rails', '~> 7.1.0' to gem 'rails', '~> 7.0.0'), everything works fine.

Unfortunately, I won't be able to follow-up further, but thought it would be nice to include official Rails 7.1 support for this gem.