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

vendor/gopkg.in/mail.v2/errors.go

@@ -0,0 +1,16 @@
+package mail
+
+import "fmt"
+
+// A SendError represents the failure to transmit a Message, detailing the cause
+// of the failure and index of the Message within a batch.
+type SendError struct {
+	// Index specifies the index of the Message within a batch.
+	Index uint
+	Cause error
+}
+
+func (err *SendError) Error() string {
+	return fmt.Sprintf("gomail: could not send email %d: %v",
+		err.Index+1, err.Cause)
+}