docs: document use of latest two Alpine releases

[MikeMcC399]

• closes Clarify Alpine version support policy #2072

Description

Alpine is added to the CONTRIBUTING > Version Updates document section to specify that the latest two Alpine Linux versions are used.

Motivation and Context

The CONTRIBUTING > Version Updates document section does not cover updates to Alpine, leaving it unclear how they should be handled. In practice, and according to #2072 (comment):

We only support 2 (Alpine releases) at the recommendation of the Official Images repo as there is a lot of build required for these images, and there is no upstream support or binaries for Alpine that would reduce that workload if more were added back

https://alpinelinux.org/releases/ states:

Each May and November we make a release branch from edge. The main repository is typically supported for 2 years and the community repository is supported until next stable release.

Branch	Date	docker-node	End-of-support
v3.23	2025-12-03	Active	2027-11-01
v3.22	2025-05-30	Active	2027-05-01
v3.21	2024-12-05	Removed	2026-11-01

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.