nixos/systemd: don't require network-online.target for multi-user.target

Previously we required network-online.target for multi-user.target. This
has made a lot of people very angry and has been widely regarded as a
bad move (or at least, very nonstandard):
https://github.com/NixOS/nixpkgs/commit/15d761a525a025de0680b62e8ab79a9d183f313d#commitcomment-128564097

This was done because of fragile tests and services declaring
dependencies on multi-user.target when they meant network-online.target.

Let's rip off the bandaid and fix our tests.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -112,6 +112,13 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 - `services.avahi.nssmdns` got split into `services.avahi.nssmdns4` and `services.avahi.nssmdns6` which enable the mDNS NSS switch for IPv4 and IPv6 respectively.
   Since most mDNS responders only register IPv4 addresses, most users want to keep the IPv6 support disabled to avoid long timeouts.
 
+- `multi-user.target` no longer depends on `network-online.target`.
+  This will potentially break services that assumed this was the case in the past.
+  This was changed for consistency with other distributions as well as improved boot times.
+
+  We have added a warning for services that are
+  `after = [ "network-online.target" ]` but do not depend on it (e.g. using `wants`).
+
 - `networking.iproute2.enable` now does not set `environment.etc."iproute2/rt_tables".text`.
 
   Setting `environment.etc."iproute2/{CONFIG_FILE_NAME}".text` will override the whole configuration file instead of appending it to the upstream configuration file.

nixos/modules/system/boot/systemd.nix

@@ -641,7 +641,6 @@ in
     systemd.services.systemd-udev-settle.restartIfChanged = false; # Causes long delays in nixos-rebuild
     systemd.targets.local-fs.unitConfig.X-StopOnReconfiguration = true;
     systemd.targets.remote-fs.unitConfig.X-StopOnReconfiguration = true;
-    systemd.targets.network-online.wantedBy = [ "multi-user.target" ];
     systemd.services.systemd-importd.environment = proxy_env;
     systemd.services.systemd-pstore.wantedBy = [ "sysinit.target" ]; # see #81138