Update RFC URLs to use html versions

Changed the URLs for RFC 4954 and RFC 4616 from plain text to HTML formats for improved readability and consistency. This adjustment does not affect the functionality but enhances the documentation quality.

auth.go

@@ -12,7 +12,7 @@ type SMTPAuthType string
 
 const (
 	// SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954.
-	// https://datatracker.ietf.org/doc/rfc4954/
+	// https://datatracker.ietf.org/doc/html/rfc4954/
 	//
 	// CRAM-MD5 is not secure by modern standards. The vulnerabilities of MD5 and the lack of
 	// advanced security features make it inappropriate for protecting sensitive communications
@@ -41,7 +41,7 @@ const (
 	SMTPAuthNoAuth SMTPAuthType = ""
 
 	// SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616.
-	// https://datatracker.ietf.org/doc/rfc4616/
+	// https://datatracker.ietf.org/doc/html/rfc4616/
 	//
 	// Since the "PLAIN" SASL authentication mechansim transmits the username and password in
 	// plaintext over the internet connection, we only allow this mechanism over a TLS secured