i18n(fr): update index.mdx (v7 banner)

[ArmandPhilippot]

Description

Updates the French translation of the homepage to add the v7 banner (see #3951).

The second sentence is a bit verbose, but I thought it might sound more natural than "Découvrez comment mettre à niveau" or "Apprenez à mettre à niveau". And it seems to be fine on small viewports:

But, I don't have a strong opinion if you prefer something else.

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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.

[HiDeoo]

Personally perfectly happy with your choices, perfect as always 🌟