Update Node.js to v21 (#503)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [node](https://togithub.com/nodejs/node) | final | major | `20-bullseye` -> `21-bullseye` |

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### Release Notes

<details>
<summary>nodejs/node (node)</summary>

### [`v21`](https://togithub.com/nodejs/node/blob/HEAD/CHANGELOG.md#2016-01-21-Version-426-Argon-LTS--TheAlphaNerd)

<a href="doc/changelogs/CHANGELOG_V4.md#4.2.6">Moved to doc/changelogs/CHANGELOG_V4.md#4.2.6</a>.

</details>

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.devcontainer/base.Dockerfile

@@ -1,5 +1,5 @@
 # [Choice] Node.js version (use -bullseye variants on local arm64/Apple Silicon): 18-bullseye, 16-bullseye, 14-bullseye, 18-buster, 16-buster, 14-buster
-ARG VARIANT=20-bullseye
+ARG VARIANT=21-bullseye
 FROM node:${VARIANT}
 
 # [Option] Install zsh