datetime: Don't use gtk_widget_show_all

It is removed in GTK+ 4
11 jobs for no-show-all in 5 minutes and 7 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #121389
build

00:01:56

 
  Test
passed #121391
asan

00:03:10

passed #121390
test

00:00:43

 
  Manual
manual #121396
allowed to fail manual
armv8
manual #121392
allowed to fail manual
flatpak
manual #121395
allowed to fail manual
msan
manual #121397
allowed to fail manual
ppc64le
manual #121393
allowed to fail manual
tsan
manual #121394
allowed to fail manual
ubsan
 
  Review
manual #121398
allowed to fail manual
review
manual #121399
allowed to fail manual
stop_review