fix(v7): make v7 upgrade guide discoverable

[ArmandPhilippot]

Description (required)

• Updates the banner to link to the v7 upgrade guide instead of the v6 one.
• Adds the v7 upgrade guide in the sidebar next to the other ones.
• Lists the v7 upgrade guide in https://v7.docs.astro.build/en/upgrade-astro/#upgrade-guides

Related issues & labels (optional)

• Suggested label: 7.0

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Tracked Files

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en/getting-started.mdx	Source changed, localizations will be marked as outdated.
en/upgrade-astro.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	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.