chore(toolchain): drop all but the latest 3.9 toolchain

[aignas]

Summary:

• drop the 3.9 versions one by one.
• add the latest astral toolchain builds.
• add a disclaimer to our changeleg about 3.9.
• update examples to use the latest 3.9 build (separate PR)
  will switch to a different python version.

Work towards #2704

[gemini-code-assist]

Summary of Changes

Hello @aignas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request streamlines the project's toolchain configuration by removing outdated Python 3.9 versions, aligning with its End-of-Life status. This cleanup reduces maintenance overhead and ensures that only actively supported toolchains are available. Additionally, a new "Unreleased" section has been introduced in the changelog to better organize and communicate forthcoming updates.

Highlights

• Toolchain Management: Obsolete Python 3.9 toolchain versions (3.9.10 through 3.9.23) have been removed from python/versions.bzl, retaining only the latest 3.9.24 version, as Python 3.9 has reached End-of-Life (EOL).
• Changelog Update: A new "Unreleased" section has been added to CHANGELOG.md to document upcoming changes, including the toolchain removals, and existing fixes have been re-categorized within this new section.

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[gemini-code-assist]

Code Review

This pull request removes older Python 3.9 toolchains, keeping only the latest one, and updates the changelog accordingly. My review focuses on the correctness of the changelog update and a minor point of confusion in the toolchain definition file. The main issue is that the changelog has been updated incorrectly by adding a duplicate 'Unreleased' section, which will likely break your release tooling. I've also pointed out a confusing comment in python/versions.bzl that seems to have been added by mistake.

[groodt]

LGTM