feat(v7): remove deprecated APIs in astro:transitions

[ArmandPhilippot]

Description (required)

• Removes the deprecated APIs from astro:transitions/client
• Adds a new section in the v7 upgrade guide

Related issues & labels (optional)

• Suggested label: 7.0, add new content

For Astro version: 7.0. See astro PR #16725.

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File	Note
en/guides/upgrade-to/v7.mdx	Localization changed, will be marked as complete.
en/reference/modules/astro-transitions.mdx	Source changed, localizations will be marked as outdated.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.

[yanthomasdev]

LGTM! Good work! 🚀