Backport !1428 “gs-app-details-page: Set update details markup as markup, not as plain text” to gnome-42
Otherwise the markup-escaped text is shown as escape sequences, instead of human-readable text.
This had been reported downstream at: https://bugzilla.redhat.com/show_bug.cgi?id=2106872
Almost trivial backport of !1428 (merged) to gnome-42. One minor cherry-pick conflict which was easy to solve.