Upgrade 8.x-3.x to 4.x
- /htmlmail.info.yml
- /htmlmail.module
- /src/Form/HtmlMailConfigurationForm.php
- /src/Form/HtmlMailTestForm.php
- /src/Helper/HtmlMailHelper.php
- /src/Plugin/Mail/HtmlMailSystem.php
- /src/Utility/HtmlMailMime.php
- /htmlmail.services.yml