tech: migrates to none deprecated mail lib

gomail is missing a maintainer so we are
switching to an active fork

ref https://github.com/go-gomail/gomail/issues/108

closes #7189

pkg/services/notifications/mailer.go

@@ -17,7 +17,7 @@ import (
 	"github.com/grafana/grafana/pkg/log"
 	m "github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/setting"
-	"gopkg.in/gomail.v2"
+	gomail "gopkg.in/mail.v2"
 )
 
 var mailQueue chan *Message