Add Alpine 3.20 variant

[jnoordsij]

Description

This PR adds a variant for Alpine 3.20 and drops the 3.18 variant at the same time.
See also #2007.

Requires docker-library/official-images#16801.

Motivation and Context

Provide latest Alpine version (see https://alpinelinux.org/posts/Alpine-3.20.0-released.html).

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.

[github-actions]

Created PR on the official-images repo (docker-library/official-images#16806). 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.