[Arista] Fix dockerd issue on Arista platforms (sonic-net#7376)

Why I did it
Recent systemd upgrade from sonic-net#7228 requires an extra cmdline parameter for dockerd to start properly.
Updating boot0 was missed as part of the systemd upgrade change.

How I did it
Just added the missing cmdline parameter in files/Aboot/boot0.j2
This change fixes sonic-net#7372

How to verify it
Boot the image and dockerd should start normally.

files/Aboot/boot0.j2

@@ -615,6 +615,9 @@ write_image_specific_cmdline() {
     # disable deterministic interface naming
     cmdline_add net.ifnames=0
 
+    # disable unified cgroup hierarchy to workaround dockerd limitation
+    cmdline_add systemd.unified_cgroup_hierarchy=0
+
     # verbosity
     cmdline_add quiet
     # Start showing systemd information from the first failing unit if any.