Remove `getSetCookie` workaround from `@keystatic/astro`

[Greenheart]

Back when the getSetCookie workaround code was added in 2023, Astro used undici 5.x. Since Astro now uses undici v6.x, it seems like we can remove the workaround for getSetCookies.

Astro package.json (with undici 5) from the time the workaround was added to @keystatic/astro https://github.com/withastro/astro/blob/71743aeca723e215c0107ecf2e111fdcc8ec7e45/packages/astro/package.json

Astro package.json from today, using undici 6.x: https://github.com/withastro/astro/blob/main/packages/astro/package.json

And even Astro v4 (legacy) uses undici v6: https://github.com/withastro/astro/blob/32b14f24cdd3cf2c963c97bca6099ef996f7aadf/packages/astro/package.json

Because undici 6 is now widely used across all Astro versions, it should be safe to make this change. Of course assuming people have somewhat updated Astro versions.

There is a small risk for a breaking change if someone were to update @keystatic/astro but not astro, so this might be worth treating as a major version bump of @keystatic/astro just to be sure.

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Note: I didn't add a changeset yet, until we decide how to version this change.

[changeset-bot[bot]]

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Latest commit: 15c5a3575b72a685977db988675e4fccc7bd5a58

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