[v6 upgrade guide] Add section for adapter major upgrade guidance

[sarah11918]

This PR adds a section to the v6 upgrade guide under "Dependency upgrades" for "Official Astro integrations"

It mentions the adapters have all had major upgrades, and links to the (currently next branch) changelog for each one to provide upgrade guidance.

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en/guides/upgrade-to/v6.mdx	Localization changed, will be marked as complete.

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.

[ArmandPhilippot]

LGTM! I checked the other integrations and I think it's safe to only list the server adapters here!