tstest/integration/vms,.github/workflows: bump Ubuntu and NixOS for VM tests + cleanup (#16098)

This PR cleans up a bunch of things in ./tstest/integration/vms:

- Bumps version of Ubuntu that's actually run from CI 20.04 -> 24.04
- Removes Ubuntu 18.04 test
- Bumps NixOS 21.05 -> 25.05

Updates#cleanup

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

tstest/integration/vms/nixos_test.go

@@ -97,7 +97,7 @@ let
 
       # Wrap tailscaled with the ip and iptables commands.
       wrapProgram $out/bin/tailscaled --prefix PATH : ${
-        lib.makeBinPath [ iproute iptables ]
+        lib.makeBinPath [ iproute2 iptables ]
       }
 
       # Install systemd unit.
@@ -127,6 +127,9 @@ in {
   # yolo, this vm can sudo freely.
   security.sudo.wheelNeedsPassword = false;
 
+  # nix considers squid insecure, but this is fine for a test.
+  nixpkgs.config.permittedInsecurePackages = [ "squid-7.0.1" ];
+
   # Enable cloud-init so we can set VM hostnames and the like the same as other
   # distros. This will also take care of SSH keys. It's pretty handy.
   services.cloud-init = {