all: use Go 1.20's errors.Join instead of our multierr package

Updates #7123

Change-Id: Ie9be6814831f661ad5636afcd51d063a0d7a907d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/osrouter/ifconfig_windows.go

@@ -18,7 +18,6 @@ import (
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tstun"
-	"tailscale.com/util/multierr"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/winnet"
 
@@ -831,5 +830,5 @@ func syncRoutes(ifc *winipcfg.IPAdapterAddresses, want []*routeData, dontDelete
 		}
 	}
 
-	return multierr.New(errs...)
+	return errors.Join(errs...)
 }