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>

cmd/derper/mesh.go

@@ -25,7 +25,7 @@ func startMesh(s *derpserver.Server) error {
 	if !s.HasMeshKey() {
 		return errors.New("--mesh-with requires --mesh-psk-file")
 	}
-	for _, hostTuple := range strings.Split(*meshWith, ",") {
+	for hostTuple := range strings.SplitSeq(*meshWith, ",") {
 		if err := startMeshWithHost(s, hostTuple); err != nil {
 			return err
 		}