Add Zod v3 → v4 codemod link to upgrade guide

[HiDeoo]

Description

This PR updates the Astro 6 upgrade guide to include a link to the community codemod for migrating from Zod 3 to Zod 4.

I went with the minimal change to add this link right now, but I guess a few questions could be raised:

• I introduce it at the end of the Zod 4 section, I guess another option could be near the beginning of the section, after linking to the Zod migration guide.
• I only went with a link, not even a command example on how to run the codemod (it's available on the linked README).
• I only mention it in the Zod 4 section, not in the "Deprecated: astro:schema and z from astro:content" section even tho it can also be relevant there. Should it be mentioned there too?

Related issues & labels

• Closes #
• Suggested label: 6.0

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[sarah11918]

Let's get this in like this for now!

• I could see posting both the upgrade link and the code mod right at the top, too (before the what should I do). Maybe something like:

### Zod 4

Astro v6.0 upgrades to [Zod 4 (CHANGELOG)](https://zod.dev/v4/changelog)), a major dependency update that may require changes to custom Zod schemas in your project. A [community Zod v3 to Zod v4 codemod](https://github.com/nicoespeon/zod-v3-to-v4) is also available.

• I think anyone dealing with upgrading the z schema stuff from other imports/exports will also end up here, so the codemod only here seems fine. BUT, we probably could update that entry to also link to the Zod 4 one? Just to be safe? It could be something like:

Astro 6.0 deprecates `astro:schema` and `z` from `astro:content` in favor of `astro/zod`. (Note that Astro 6.0 also [upgrades to Zod 4](#zod-4) and may require additional upgrades to your project.)

[HiDeoo]

Let's get this in like this for now!

Sounds like a good call to me (and also why I initially did not go with adding it to multiple places, etc but kept it as simple as possible) 👍

[sarah11918]

Approving as is, or if you'd like to include any extras as shown in my comment either now, or later!