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>

util/linuxfw/iptables.go

@@ -18,7 +18,6 @@ import (
 
 	"github.com/coreos/go-iptables/iptables"
 	"tailscale.com/types/logger"
-	"tailscale.com/util/multierr"
 	"tailscale.com/version/distro"
 )
 
@@ -67,7 +66,7 @@ func detectIptables() (int, error) {
 	default:
 		return 0, FWModeNotSupportedError{
 			Mode: FirewallModeIPTables,
-			Err:  fmt.Errorf("iptables command run fail: %w", multierr.New(err, ip6err)),
+			Err:  fmt.Errorf("iptables command run fail: %w", errors.Join(err, ip6err)),
 		}
 	}
 
@@ -232,5 +231,5 @@ func clearRules(proto iptables.Protocol, logf logger.Logf) error {
 		errs = append(errs, err)
 	}
 
-	return multierr.New(errs...)
+	return errors.Join(errs...)
 }

util/winutil/restartmgr_windows.go

@@ -19,7 +19,6 @@ import (
 	"github.com/dblohm7/wingoes"
 	"golang.org/x/sys/windows"
 	"tailscale.com/types/logger"
-	"tailscale.com/util/multierr"
 )
 
 var (
@@ -538,7 +537,7 @@ func (rps RestartableProcesses) Terminate(logf logger.Logf, exitCode uint32, tim
 	}
 
 	if len(errs) != 0 {
-		return multierr.New(errs...)
+		return errors.Join(errs...)
 	}
 	return nil
 }