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