Added avif to translation string

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What it says on the tin. :)

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PR Summary

• Update to Accepted Image File Types
  In our language resources file general.php, we have extended the list of accepted image file types. Previously we only supported 'jpg', 'webp', 'png', 'gif', and 'svg'. Now we have added the 'avif' file type to the list, allowing for more flexible user uploads. This update does not affect the maximum upload size allowed, maintaining the same upload capacity.