[wallpapers] Use more high-quality JPEGs for Next

Summary:
Due to filesize concerns, we optimized the Volna wallpaper to be JPEG compressed at 90% quality.

JPEGs seems like the right way to go, but at 90% there is a noticeable loss of sharpness and JPEG artifacts appear, both of which are more prominent the smaller the resolution.

I used the same tool (squoosh) to generate more high quality JPEGs, paying attention to how each resolution responded to a certain quality setting.

In the end I had to use values between 94 and 98 in order to ensure there is no noticeable difference with the original PNGs.

This brings up the total file size to 17.4 MiB, which I think is still fine, especially given that this is our default wallpaper for an LTS release.

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27045