i18n(fr): add upgrade-to/v6.mdx

[ArmandPhilippot]

Description (required)

Translates in French the new upgrade-to/v6 guide added in #12322

Related issues & labels (optional)

• Suggested label: i18n

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Tracked Files

File	Note
fr/guides/upgrade-to/v6.mdx	Localization added, will be marked as complete.
fr/reference/adapter-reference.mdx	Localization changed, will be marked as complete. 🔄️
fr/reference/errors/content-collection-invalid-type.mdx	Localization changed, will be marked as complete.
fr/reference/errors/legacy-content-config-error.mdx	Localization changed, will be marked as complete.
fr/reference/modules/astro-transitions.mdx	Localization changed, will be marked as complete.

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.

[ld-web]

LGTM! Just a few comments and suggestions.

[ArmandPhilippot]

Thanks a lot for the review! I know this one was pretty long... Glad to see there are not many issues. I applied all your suggestions so it looks like we can finally ship it! 🎉