network: unify IPv4ProxyARPAddress= and IPv6ProxyNDPAddress=

It is not necessary to provide dedicated settings for IPv4 and IPv6
proxy neighbor entries. Let's unify them.

The IPv4ProxyARPAddress= setting was introduced after v261. Hence, we
can freely rename it.

test/test-network/conf/25-ipv4-proxy-arp-disabled.network

@@ -4,7 +4,7 @@ Name=dummy98
 
 [Network]
 IPv4ProxyARP=no
-IPv4ProxyARPAddress=192.0.2.1
-IPv4ProxyARPAddress=192.0.2.2
+ProxyNeighbor=192.0.2.1
+ProxyNeighbor=192.0.2.2
 IPv6AcceptRA=no
 Address=10.0.0.1/24

test/test-network/conf/25-ipv4-proxy-arp.network

@@ -3,10 +3,10 @@
 Name=dummy98
 
 [Network]
-IPv4ProxyARPAddress=192.0.2.1
-IPv4ProxyARPAddress=192.0.2.2
-IPv4ProxyARPAddress=192.0.2.3
-IPv4ProxyARPAddress=192.0.2.4
-IPv4ProxyARPAddress=192.0.2.5
+ProxyNeighbor=192.0.2.1
+ProxyNeighbor=192.0.2.2
+ProxyNeighbor=192.0.2.3
+ProxyNeighbor=192.0.2.4
+ProxyNeighbor=192.0.2.5
 IPv6AcceptRA=no
 Address=10.0.0.1/24

test/test-network/conf/25-ipv6-proxy-ndp.network

@@ -3,11 +3,11 @@
 Name=dummy98
 
 [Network]
-IPv6ProxyNDPAddress=2607:5300:203:5215:5::1
-IPv6ProxyNDPAddress=2607:5300:203:5215:4::1
-IPv6ProxyNDPAddress=2607:5300:203:5215:3::1
-IPv6ProxyNDPAddress=2607:5300:203:5215:2::1
-IPv6ProxyNDPAddress=2607:5300:203:5215:1::1
+ProxyNeighbor=2607:5300:203:5215:5::1
+ProxyNeighbor=2607:5300:203:5215:4::1
+ProxyNeighbor=2607:5300:203:5215:3::1
+ProxyNeighbor=2607:5300:203:5215:2::1
+ProxyNeighbor=2607:5300:203:5215:1::1
 IPv6AcceptRA=no
 Address=66.70.129.136/32
 Address=66.70.129.142/32

test/test-network/systemd-networkd-tests.py

@@ -5384,10 +5384,10 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         for i in range(1, 6):
             self.assertRegex(output, f'192.0.2.{i} *proxy')
 
-        # IPv4ProxyARPAddress= implies IPv4ProxyARP=yes, mirroring IPv6ProxyNDPAddress=.
+        # When a proxy ARP address is specified, IPv4ProxyARP=yes is implied.
         self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp', '1')
 
-        # Explicit IPv4ProxyARP=no must suppress all IPv4ProxyARPAddress= entries and
+        # Explicit IPv4ProxyARP=no must suppress all IPv4 proxy ARP address entries and
         # must not force proxy_arp=1. The module is add-only (no reconcile/remove pass),
         # so phase 2 starts from a clean interface: stop networkd, delete the dummy to
         # flush the kernel neighbor-proxy table, swap the .network file, and restart.