解决Spring Boot Mail无法发送HTML格式的邮件

Spring Boot版本：2.6.2

查阅spring-boot-starter-mail源码的MimeMessageHelper.setText方法，发现有个Boolean类型参数控制是否是HTML格式，方法签名如下：

public void setText(String text, boolean html) throws MessagingException

MimeMessage mailMessage = mailSender.createMimeMessage();
MimeMessageHelper messageHelper = new MimeMessageHelper(mailMessage, Boolean.TRUE, StandardCharsets.UTF_8.toString());
messageHelper.setSubject(subject);
// Boolean.TRUE设置HTML格式文本
messageHelper.setText(loadTemplate(message.getMessage()), Boolean.TRUE);
messageHelper.setFrom(from);
messageHelper.setTo(message.getTo());
mailSender.send(mailMessage);