backup: Tweak header-suffix buttons from some AdwPreferencesGroups
Use the .flat style in buttons that are header suffixes of a AdwPreferencesGroup. This is already the case for the "Refresh" button from the "Archives" preferences group located in the "Archives" view.
Also, use valign=center to make these buttons better aligned with the title and description from their preferences groups.
Edited by Automeris naranja