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>

types/netmap/nodemut_test.go

@@ -34,7 +34,7 @@ func TestMapResponseContainsNonPatchFields(t *testing.T) {
 			return reflect.ValueOf(int64(1)).Convert(t)
 		case reflect.Slice:
 			return reflect.MakeSlice(t, 1, 1)
-		case reflect.Ptr:
+		case reflect.Pointer:
 			return reflect.New(t.Elem())
 		case reflect.Map:
 			return reflect.MakeMap(t)
@@ -43,8 +43,7 @@ func TestMapResponseContainsNonPatchFields(t *testing.T) {
 	}
 
 	rt := reflect.TypeFor[tailcfg.MapResponse]()
-	for i := range rt.NumField() {
-		f := rt.Field(i)
+	for f := range rt.Fields() {
 
 		var want bool
 		switch f.Name {