Update the Twitter logo to X

Hi everyone,

I've noticed that we still use the Twitter logo on the footer.
We should switch to the `X` one.

<img width="231" alt="Screenshot 2024-05-13 at 16 43 31" src="https://github.com/openfoodfacts/openfoodfacts-server/assets/246838/42127f3f-0a2a-4d73-a02d-dd494bbb5821">

https://fr.m.wikipedia.org/wiki/Fichier:X_logo_2023.svg