feat: Node.js 21.0.0

[jnoordsij]

Description

Adds the v21 major line. Modelled after #1882 and #1792.
See also https://nodejs.org/en/blog/release/v21.0.0.

Motivation and Context

Testing Details

Example Output(if appropriate)

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[jnoordsij]

Force-pushed this to include changes from #1901 and removed buster variant per review.

[SimenB]

thanks!

[SimenB]

@nodejs/docker any reason not to land this?

[github-actions]

Created PR on the official-images repo (docker-library/official-images#15581). See https://github.com/docker-library/faq#an-images-source-changed-in-git-now-what if you are wondering when it will be available on the Docker Hub.