Problem of html in mail with symfony mailer

Problem/Motivation

Hello, I use symfony mailer to send my emails, the mail sent with hook_mail are supposed to be in plain_text format.
So this is the body of my mail sent by symfony mailer:

Hello {{username}}, This is the OTP you will use to login: {{otp}}

Steps to reproduce

Generate an OTP email with symfony mailer

Proposed resolution

We need to create a Markup object with the string to let symfony mailer know this is already html. I made a patch.