Plaintext emails rendered from templates contain escaping

### Environment SaaS (https://sentry.io/) ### Steps to Reproduce 1. Send any email using a text (not HTML) template, where one of the context variables being rendered contains special characters (&,<,>,',etc) ### Expected Result 1. Email renders as straightforward plaintext without any form of escaping ### Actual Result Email is rendered with inappropriate escaping that's not needed ### Product Area Other ### Link _No response_ ### DSN _No response_ ### Version _No response_