all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/tsnet.go

@@ -466,9 +466,7 @@ func (s *Server) close() {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	var wg sync.WaitGroup
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	wg.Go(func() {
 		// Perform a best-effort final flush.
 		if s.logtail != nil {
 			s.logtail.Shutdown(ctx)
@@ -476,14 +474,12 @@ func (s *Server) close() {
 		if s.logbuffer != nil {
 			s.logbuffer.Close()
 		}
-	}()
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	})
+	wg.Go(func() {
 		if s.localAPIServer != nil {
 			s.localAPIServer.Shutdown(ctx)
 		}
-	}()
+	})
 
 	if s.shutdownCancel != nil {
 		s.shutdownCancel()

tsnet/tsnet_test.go

@@ -2598,7 +2598,7 @@ func buildDNSQuery(name string, srcIP netip.Addr) []byte {
 		0x00, 0x01, // QDCOUNT: 1
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // ANCOUNT, NSCOUNT, ARCOUNT
 	}
-	for _, label := range strings.Split(name, ".") {
+	for label := range strings.SplitSeq(name, ".") {
 		dns = append(dns, byte(len(label)))
 		dns = append(dns, label...)
 	}