Mail: Update preview's iframe height to match its content

webkitgtk 2.37.90 recently appeared as an update for Fedora 37 (and Rawhide). If you update to this version, message display in Evolution breaks (you just see a small blank rectangle, the message is not displayed at all). No errors are shown at a console. Downgrading back to webkitgtk 2.37.1 fixes things again.

Not sure if the bug is in webkitgtk or Evolution, filing here for now. Rebuilding Evolution against the newer webkitgtk does not help (I didn't expect it to since there was no soname change and no errors indicating a symbol issue, but tried just in case).