Remove the original changelog (#15123)

This removes the original changelog now that it is located in the cargo
book.

This places a notice in the original location that it has moved to a new
place. There are a few other alternatives we could consider:
* Keep the headings but replace the content with a link. This would make
it a little easier for someone to jump to the exact position in the new
page. However, it looks a little messy. Each section could be something
like:

> This content has moved to
[CHANGELOG.md#186-2025-04-03](https://doc.rust-lang.org/cargo/nightly/CHANGELOG.md#186-2025-04-03)

* Keep the content, with a notice at the top that the page is no longer
being updated. I was a little uncomfortable with this, since I wasn't
sure people would see the notice, it duplicates some content, and still
leaves the potential problems of GitHub not rendering the page. If we
cut some content, it ensures the page still renders reliably.
* As a variant, we could delete the content but keep the last 1 or 2
releases since those are probably what people will be looking for.

Opening as draft until #15119 is
published on nightly.