cubicweb-folder #5198700 do not rely on cubicweb.web.httpcache.GMTOFFSET which is going away [validation pending]

see #2154655

prioritynormal
typebug
done in1.10.0
load0.500
load left0.000
closed by#ef157c73c3e9 [views] Drop last_modified method from Folder primary view
patch[views] Drop last_modified method from Folder primary view [applied]