Add .txt format support to changelog API

What does this MR do and why?

Every time I want to preview what the next release will include for my project I hit https://gitlab.com/api/v4/projects/56614952/repository/changelog?version=v999.999.999 and see something like:

{"notes":"## 999.999.999 (2026-05-22)\n\n### Maintenance (6 changes)\n\n- [Update dependency...

Then I have to copy/paste into VSCode, trim the JSON then find & replace \\n with \n to see the plain text.

This MR adds .txt format support to the changelog GET endpoint, using Grape built-in format routing (the same pattern used by the repository archive endpoint). Appending .txt to the URL returns the changelog as plain text Markdown instead of JSON.

GET /projects/:id/repository/changelog.txt?version=1.0.0

References

Similar pattern: Retrieve file archive from a repository uses [.format] suffix for response format control.

How to set up and validate locally

1. Create a tag & release
2. Push a commit with a changelog trailer like:

   Did a thing
   
   Changelog: Feature

3. Hit http://gdk.test:3000/api/v4/projects/19/repository/changelog?version=v999.999.999 and see the JSON changelog
4. Hit http://gdk.test:3000/api/v4/projects/19/repository/changelog.txt?version=v999.999.999 and see the plain text changelog

MR acceptance checklist

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