hostnamed: use /proc/device-tree to get chassis type

From https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-firmware-ofw
---
Userspace must not use the /sys/firmware/devicetree/base
path directly, but instead should follow /proc/device-tree
symlink. It is possible that the absolute path will change
in the future, but the symlink is the stable ABI.
---

Addresses the comment https://github.com/systemd/systemd/pull/20731#discussion_r744095262.

src/hostname/hostnamed.c

@@ -302,7 +302,7 @@ try_acpi:
         }
 
 try_devicetree:
-        r = read_one_line_file("/sys/firmware/devicetree/base/chassis-type", &type);
+        r = read_one_line_file("/proc/device-tree/chassis-type", &type);
         if (r < 0) {
                 log_debug_errno(r, "Failed to read device-tree chassis type, ignoring: %m");
                 return NULL;