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/viewer/viewer.go

@@ -500,8 +500,7 @@ func genView(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named, fie
 		}
 		writeTemplateWithComment("unsupportedField", fname)
 	}
-	for i := range typ.NumMethods() {
-		f := typ.Method(i)
+	for f := range typ.Methods() {
 		if !f.Exported() {
 			continue
 		}
@@ -720,7 +719,7 @@ func main() {
 	fieldComments := getFieldComments(pkg.Syntax)
 
 	cloneOnlyType := map[string]bool{}
-	for _, t := range strings.Split(*flagCloneOnlyTypes, ",") {
+	for t := range strings.SplitSeq(*flagCloneOnlyTypes, ",") {
 		cloneOnlyType[t] = true
 	}