Port to the new "description" property of AdwPreferencesPage

Ports preferences page description to use the new "description" property of the page, instead of the one of an empty or the only preferences group. The new property is implemented in libadwaita!828 (merged), so the MR also updates libadwaita version to 1.4.alpha. This update causes CI to break, even though it also includes changes to the .gitlab-ci.yml — they seem to be ignored, see !1754 (comment 1711652). Bluetooth panel is left out from this MR because it resides in a separate library and the change would go against the new mockup. Panels that do not use AdwPreferencesPage are also left out.