Dropped -provider icon and proper action text.

The formerly used icon is no longer provided by icon sets, so a switch
to an existing icon is needed.
See https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_428725
and https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_428726

Also capitalized action text.
See https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_428723
and https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_428724