Add #[AllowDynamicProperties] attribute to avoid the deprecation notice.
See https://php.watch/versions/8.2/dynamic-properties-deprecated
# ERROR 8192: Creation of dynamic property MimeMailParser::$parts is deprecated at [/var/www/html/phorge/phorge/externals/mimemailparser/MimeMailParser.class.php:137] # #0 MimeMailParser::parse() called at [<phorge>/externals/mimemailparser/MimeMailParser.class.php:126]
Closes T15765