Split RELEASES.md into new "2021 edition" file

To ensure that splitting the file didn't break formatting, I ran the
following commands:

```sh
pulldown-cmark <RELEASES_2021_EDITION.md >rendered_2021.html
pulldown-cmark <RELEASES.md >rendered.html
diff rendered.html rendered_2021.html | rg "^>"
```

At first the diff included multiple new lines in `rendered_2021.html`,
indicating links that weren't rendered to HTML because their target was
missing. After I added those targets, but diff was remove-only.

The general policy for RELEASES files would be to update the _2021
file until edition 2024 comes, then we'd create a RELEASES_2021_EDITION
file (also linked from the RELEASES.md file), and so on.

We could optionally add a RELEASES_2018_EDITION and/or a RELEASES_OLD
file, but that decision is outside the scope of this commit.