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]

@sxa / @nschonni

Could we possibly move this along? Without this PR the README is now inaccurate, as it currently still states:

All of the images contain pre-installed versions of node, npm, and Yarn v1 Classic.