nixos/networkd: remove CriticalConnection= fields in favor of KeepConnection

Systemd upstream has deprecated CriticalConnection with v244 in favor of
KeepConnection as that seems to be more flexible:

  The CriticalConnection= setting in .network files is now deprecated,
  and replaced by a new KeepConfiguration= setting which allows more
  detailed configuration of the IP configuration to keep in place.

nixos/doc/manual/release-notes/rl-2009.xml

@@ -290,6 +290,16 @@ environment.systemPackages = [
      has been set to <literal>true</literal>.
     </para>
    </listitem>
+   <listitem>
+    <para>
+      The <literal>systemd-networkd</literal> option
+      <literal>systemd.network.networks.&lt;name&gt;.dhcp.CriticalConnection</literal>
+      has been removed following upstream systemd's deprecation of the same. It is recommended to use
+      <literal>systemd.network.networks.&lt;name&gt;.networkConfig.KeepConfiguration</literal> instead.
+      See <citerefentry><refentrytitle>systemd.network</refentrytitle>
+      <manvolnum>5</manvolnum></citerefentry> for details.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/system/boot/networkd.nix

@@ -205,7 +205,7 @@ let
       "IPv6HopLimit" "IPv4ProxyARP" "IPv6ProxyNDP" "IPv6ProxyNDPAddress"
       "IPv6PrefixDelegation" "IPv6MTUBytes" "Bridge" "Bond" "VRF" "VLAN"
       "IPVLAN" "MACVLAN" "VXLAN" "Tunnel" "ActiveSlave" "PrimarySlave"
-      "ConfigureWithoutCarrier" "Xfrm"
+      "ConfigureWithoutCarrier" "Xfrm" "KeepConfiguration"
     ])
     # Note: For DHCP the values both, none, v4, v6 are deprecated
     (assertValueOneOf "DHCP" ["yes" "no" "ipv4" "ipv6" "both" "none" "v4" "v6"])
@@ -228,6 +228,7 @@ let
     (assertValueOneOf "ActiveSlave" boolValues)
     (assertValueOneOf "PrimarySlave" boolValues)
     (assertValueOneOf "ConfigureWithoutCarrier" boolValues)
+    (assertValueOneOf "KeepConfiguration" (boolValues ++ ["static" "dhcp-on-stop" "dhcp"]))
   ];
 
   checkAddress = checkUnitConfig "Address" [
@@ -277,7 +278,7 @@ let
   checkDhcp = checkUnitConfig "DHCP" [
     (assertOnlyFields [
       "UseDNS" "RoutesToDNS" "UseNTP" "UseMTU" "Anonymize" "SendHostname" "UseHostname"
-      "Hostname" "UseDomains" "UseRoutes" "UseTimezone" "CriticalConnection"
+      "Hostname" "UseDomains" "UseRoutes" "UseTimezone"
       "ClientIdentifier" "VendorClassIdentifier" "UserClass" "DUIDType"
       "DUIDRawData" "IAID" "RequestBroadcast" "RouteMetric" "RouteTable"
       "ListenPort" "RapidCommit"
@@ -292,7 +293,6 @@ let
     (assertValueOneOf "UseDomains" ["yes" "no" "route"])
     (assertValueOneOf "UseRoutes" boolValues)
     (assertValueOneOf "UseTimezone" boolValues)
-    (assertValueOneOf "CriticalConnection" boolValues)
     (assertValueOneOf "RequestBroadcast" boolValues)
     (assertInt "RouteTable")
     (assertMinimum "RouteTable" 0)