Add TLS/STARTTLS support for sending verification emails

[LTSlw]

Currently, the application’s email sending functionality for verification messages does not support TLS/STARTTLS. This limits secure communication with modern SMTP servers, as many providers require encrypted connections to accept messages. Without TLS/STARTTLS, verification emails may fail to deliver or risk being intercepted.

Related code:

torrust-index/src/mailer.rs

Lines 70 to 91 in f9c17f3

	async fn get_mailer(cfg: &Configuration) -> Mailer {
	let settings = cfg.settings.read().await;
	if !settings.mail.smtp.credentials.username.is_empty() && !settings.mail.smtp.credentials.password.is_empty() {
	// SMTP authentication
	let creds = Credentials::new(
	settings.mail.smtp.credentials.username.clone(),
	settings.mail.smtp.credentials.password.clone(),
	);
	AsyncSmtpTransport::<Tokio1Executor>::builder_dangerous(&settings.mail.smtp.server)
	.port(settings.mail.smtp.port)
	.credentials(creds)
	.authentication(vec![Mechanism::Login, Mechanism::Xoauth2, Mechanism::Plain])
	.build()
	} else {
	// SMTP without authentication
	AsyncSmtpTransport::<Tokio1Executor>::builder_dangerous(&settings.mail.smtp.server)
	.port(settings.mail.smtp.port)
	.build()
	}
	}

Doc ref: https://docs.rs/lettre/latest/lettre/transport/smtp/struct.AsyncSmtpTransport.html#method.builder_dangerous