HACKING.md: Rename margin-left to margin-start

Left and right margin names are not RTL friendly and will be dropped in
GTK+ 4.
3 jobs for margin-start-end in 7 minutes and 17 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #3817
librem5
build-debian-clang

00:02:48

passed #3816
librem5
build-debian-gcc

00:02:12

24.8%
passed #3818
librem5
build-debian-package

00:02:16