As part of https://core.trac.wordpress.org/ticket/55443, it was recently decided to change the default behavior of the WebP implementation back to generating only WebP versions for the sub-sizes of a JPEG upload, rather than both WebP and JPEG versions for every size.

We should align with this in the plugin, i.e. adjust the default value for the webp_uploads_upload_image_mime_transforms filter, so that both image/jpeg and image/webp result in only image/webp.

While the short-term future of the WebP feature is unclear, we should definitely align the plugin implementation with the latest decision made for the core side, as that is the safer default.