docs: remove Yarn upgrade/downgrade best practices

[MikeMcC399]

Description

Remove the outdated section Upgrading/downgrading Yarn from the Docker and Node.js Best Practices document.

Motivation and Context

The section describes Upgrading/downgrading Yarn for Node.js 6 and Yarn 1.16.0, referring to an end-of-life Node.js version and the unsupported Yarn v1 Classic module.

Yarn v1 is no longer being bundled for Docker images based on Node.js >=26.

For new Docker images based on Node.js <=25, the Yarn v1 version is set to yarn@1.22.22 (released March 2024) and is not expected to change. The version itself is frozen and no longer supported.

The instructions can no longer be maintained, since Yarn v1 Classic is now unsupported. The instructions are outdated and only partially work.

In the meantime yarn@1.22.22 is the most viable Yarn v1 Classic version, so there would be no need to downgrade. It's also the highest version for Yarn v1, so there is nothing to upgrade to, apart from migrating to a higher major version, such as Yarn Modern (v4), which is out-of-scope for this description in any case.

Testing Details

N/A - documentation change only

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.