networkd: downgrade error for non-system users/groups to a warning

This reverts (in spirit) "network/tuntap: deny from owning Tun/Tap interfaces",
commit 940441b44c.

Justification similar as in the previous commit. The check is only
partially connected to the intended purpose and breaks backwards compat
without a sufficient reason.

Alternative fix for #37279.

catalog/systemd.catalog.in

@@ -989,15 +989,15 @@ will fail.
 Subject: Non-system user or group used for device ownership
 Defined-By: systemd
 Support: %SUPPORT_URL%
-Documentation: man:systemd(1) systemd-udevd(8)
+Documentation: man:systemd(1) systemd-udevd(8) systemd-networkd(8)
 
-The ownership of a device managed by systemd-udevd is assigned to a "regular"
-(non-system) user or group. This is currently allowed for compatibility, but is
-deprecated and discouraged. Ownership of a device node grants the privileges to
-change ACLs, the group, access mode, or set labels or extended attributes,
-which creates a conflict of management, because both udev and the user are in
-power to change these attributes. In addition, device nodes appear early in
-boot, while regular users may appear only later.
+The ownership of a device managed by systemd-udevd or systemd-networkd is
+assigned to a "regular" (non-system) user or group. This is currently allowed
+for compatibility, but is deprecated and discouraged. Ownership of a device
+node grants the privileges to change ACLs, the group, access mode, or set
+labels or extended attributes, which creates a conflict of management, because
+both udev and the user are in power to change these attributes. In addition,
+device nodes appear early in boot, while regular users may appear only later.
 
 For devices managed by systemd-udevd, it is instead recommended to use the
 "uaccess"/"xaccess" mechanisms to grant limited and temporary access to device