docs: remove reference to EOL Alpine 3.18

[MikeMcC399]

Description

Remove the reference to Alpine 3.18 from the README > node:alpine section.

Motivation and Context

The README > node:alpine section contains the advice:

• For Alpine v3.18 and earlier, adding the libc6-compat package in your Dockerfile is recommended: apk add --no-cache libc6-compat

Alpine 3.18 reached its end-of-support transition on 2025-05-09

PR #2085 previously dropped the Alpine 3.18 variant in May 2024 from Node.js Docker builds.

Testing Details

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.