Close Python 3.12 migration

[jakirkham]

A year and one month ago we started the Python 3.12 migration ( #4914 ). Since then 97% of feedstocks have been migrated or have an open Python 3.12 migration PR

About 5 months ago we opted to soft-close the Python 3.12 migration ( #5851 ). As a result feedstocks have Python 3.12 added to their build matrix even without a migrator (unless they explicitly exclude it). Thus likely many more already have Python 3.12 enabled

Almost 2 months ago we started the Python 3.13 migration

Given all this, it seems like it is time to close the Python 3.12 migrator. So this does that. Note that the global matrix already has Python 3.12. So there are no changed needed there

Closes #5892

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Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[jakirkham]

Thoughts @conda-forge/core ?