Add TYPO3 14 support

[dmitryd]

This pull request targets 12.x because develop is currently behind 12.x and causes merge conflicts.

This pull request is based on the TYPO3 v13 work from https://github.com/beechit/direct_mail_v13. Many thanks to the maintainers of that fork for providing TYPO3 v13 support.

Since TYPO3 v13 support has not been merged yet, this PR may mainly serve as a reference for integrators who need TYPO3 v14 support today. The branch can also be used directly in a local project.

Example composer.json configuration:

{
  "repositories": [
    {
      "type": "git",
      "url": "https://github.com/dmitryd/direct_mail.git"
    }
  ],
  "require": {
    "directmailteam/direct-mail": "dev-typo3-14-clean"
  }
}

Special thanks to snowflake.ch and koordination.ch for sponsoring this update! 🎉

Refs: #548