Split RELEASES.md into new "2021 edition" file

[PoignardAzur]

Split RELEASES.md into new RELEASES_2021_EDITION.md file

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

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. Whether we should do that is outside the scope of this PR
(and I'd appreciate if we could avoid bikeshedding it here).

[rustbot]

r? @Mark-Simulacrum

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[PoignardAzur]

The following files have no copyright and licensing information:
*  ../RELEASES_2021_EDITION.md

Uh. I guess there's some ignore file I should add this to?

[PoignardAzur]

Apparently the reuse tool is mad at me because RELEASES_2021_EDITION.md doesn't have a license header... but RELEASES.md doesn't have one either and it's fine?

It could be that RELEASE.md is in an ignore list, either hardcoded in reuse or somewhere in the repo; but either way I haven't found it.

Anybody know what's going on?

[PoignardAzur]

I've added RELEASES_2021_EDITION.md to the .reuse/dep5 file. Should be good now.

[rust-log-analyzer]

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

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GITHUB_EVENT_NAME=pull_request
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GITHUB_STATE=/home/runner/work/_temp/_runner_file_commands/save_state_af5fbf07-1bda-48cf-8308-96af3b61046c
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GITHUB_TRIGGERING_ACTOR=PoignardAzur
GITHUB_WORKFLOW_REF=rust-lang/rust/.github/workflows/ci.yml@refs/pull/118280/merge
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GITHUB_WORKSPACE=/home/runner/work/rust/rust
GOROOT_1_19_X64=/opt/hostedtoolcache/go/1.19.13/x64
---
##[group]Building stage0 tool linkchecker (x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.11s
##[endgroup]
##[group]Testing stage0 Linkcheck (x86_64-unknown-linux-gnu)
releases.html:33: broken link - `RELEASES_2021_EDITION.md`
number of HTML files scanned: 34855
number of HTML redirects found: 10552
number of links checked: 3013203
number of links ignored due to external: 65341

[PoignardAzur]

Not sure how to fix this error. I could change the link to https://github.com/rust-lang/rust/blob/master/RELEASES_2021_EDITION.md, but that path doesn't exist yet.

I could remove the link altogether until the PR is merged, then add it back in a following PR.

[Mark-Simulacrum]

I'm going to close this for now -- the current planned solution is rendering the release notes ourselves (via #117888). That is already merged & working, and once it hits stable we'll probably update our automation to link to that instead of the GitHub rendered view for future releases.