Add config option to use text-only emails

### Is your feature request related to a problem?

Not really, I just don't want to send out HTML emails to my users.

### Describe the solution you'd like

A config option that just disables the HTML content of emails and just includes the text-only part.

### Describe alternatives that you've considered

Manually apply a patch to authentik, that just hardcoded removes the html emails.

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating the email configuration and rendering paths, then verify that a new setting omits HTML while retaining the text-only content and that the default behavior remains unchanged.

Written by the indexing model from the issue text.