docs: reword Yarn v1 bundling descriptions for Node.js 26 release

[MikeMcC399]

Description

Reword bundling descriptions regarding Yarn v1 in the documents:

• README
• CONTRIBUTING

Motivation and Context

The README document, including the section Yarn v1 Classic bundling is written as a statement of plans and preparation for Node.js 26 and above.

With the release of Node.js 26, the implementation dropping Yarn v1 Classic is now realized.

Testing Details

N/A

Example Output (if appropriate)

N/A

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.

[MikeMcC399]

Preview

• https://github.com/MikeMcC399/docker-node/blob/docs/yarn-v1-node-26-release/README.md#image-variants
• https://github.com/MikeMcC399/docker-node/blob/docs/yarn-v1-node-26-release/README.md#yarn-v1-classic-bundling
• https://github.com/MikeMcC399/docker-node/blob/docs/yarn-v1-node-26-release/README.md#yarn-v1-classic-bundling

[MikeMcC399]

Requires #2484 to pass link checking

[MikeMcC399]

Rebased to fix markdown-link-check issues

[sxa]

Would it be better to say "is not longer included" with Node 26 and above as opposed to saying "it is removed"? Otherwise LGTM

[MikeMcC399]

I wrote it like that because this is the documentation for contributors who I expect to be more concerned about internal processes and script workings.

The templates like Dockerfile-debian.template include a section which installs Yarn. When the update.sh script runs, it removes this Yarn installation section when it constructs a Dockerfile like 26/trixie/Dockerfile for a Node.js version >=26. The net result is that the Dockerfile doesn't install Yarn.

The README document words it differently and doesn't talk about anything being removed because only the end-result is relevant for the end-consumer of the Docker images.

It would be possible to take out that level of detail here if you think it would be confusing. I'd prefer to leave it as it is though to give possible contributors better insight into internals. Perhaps I should just add in the description I've given to you as explanation?