Fix link in plain-text re-request MR review mails

What does this MR do and why?

Tiny follow-up to Added backend for requesting a new review (!52321 - merged)

In the plain-text email template merge_request_reference_link() was used, which outputs the HTML link – not just the raw URL. This causes weirdness in some email clients.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Disable multipart emails
2. Re-request review on an MR
3. Observe HTML output in plaintext email

MR acceptance checklist

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