Remove ViewTransitions in v6

[ellielok]

Description (required)

Add docs for the upcoming v6 changes for pull request withastro/astro#14400

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en/guides/upgrade-to/v6.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

This is great @ellielok ! Thank you for sticking so closely to the original format from the previous guide. It made this really easy to review and had all the right info we needed.

I just updated the text a little bit to focus on going from "existed to no longer exists" since the kind of change is a tiny bit different now. But this is exactly what we need!

[florian-lefebvre]

Thank you!