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