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>

net/udprelay/server.go

@@ -977,7 +977,7 @@ func (e ErrServerNotReady) Error() string {
 // For now, we favor simplicity and reducing VNI re-use over more complex
 // ephemeral port (VNI) selection algorithms.
 func (s *Server) getNextVNILocked() (uint32, error) {
-	for i := uint32(0); i < totalPossibleVNI; i++ {
+	for range totalPossibleVNI {
 		vni := s.nextVNI
 		if vni == maxVNI {
 			s.nextVNI = minVNI

net/udprelay/server_test.go

@@ -265,7 +265,7 @@ func TestServer(t *testing.T) {
 			tcB := newTestClient(t, endpoint.VNI, tcBServerEndpointAddr, discoB, discoA.Public(), endpoint.ServerDisco)
 			defer tcB.close()
 
-			for i := 0; i < 2; i++ {
+			for range 2 {
 				// We handshake both clients twice to guarantee server-side
 				// packet reading goroutines, which are independent across
 				// address families, have seen an answer from both clients
@@ -345,7 +345,7 @@ func TestServer_getNextVNILocked(t *testing.T) {
 	s := &Server{
 		nextVNI: minVNI,
 	}
-	for i := uint64(0); i < uint64(totalPossibleVNI); i++ {
+	for range uint64(totalPossibleVNI) {
 		vni, err := s.getNextVNILocked()
 		if err != nil { // using quicktest here triples test time
 			t.Fatal(err)