Bring back Astro v6

[delucis]

• This PR reverts the revert we did in Revert update to Astro 6 #2217
• It updates Astro & other deps to v6

Browser Test Checklist

I have tested this PR on at least three of the following browsers:

• Chrome / Chromium
• Firefox
• Android Firefox
• Safari
• iOS Safari

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[sarah11918]

Noting that an animated theme thumbnail is visible at https://deploy-preview-2223--astro-www-2.netlify.app/themes/ (And I believe we had previously disallowed that, so that fix may have been lost in the initial upgrade to v6?)

[ArmandPhilippot]

Tested again with Firefox and Chromium on Linux, and nothing seems broken! 🚀

Noting that an animated theme thumbnail is visible at https://deploy-preview-2223--astro-www-2.netlify.app/themes/ (And I believe we had previously disallowed that, so that fix may have been lost in the initial upgrade to v6?)

It seems to also be the case on the live website so it doesn't seem related to v6? Now, I don't have the history so maybe there was a regression at some point, but it doesn't seem related to this upgrade.

[delucis]

Noting that an animated theme thumbnail is visible at deploy-preview-2223--astro-www-2.netlify.app/themes (And I believe we had previously disallowed that, so that fix may have been lost in the initial upgrade to v6?)

This has been the case for some time, so not related to this PR. We’ll have to dig into it separately 👍