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>

util/linuxfw/fake.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -60,10 +61,8 @@ func (n *fakeIPTables) Append(table, chain string, args ...string) error {
 func (n *fakeIPTables) Exists(table, chain string, args ...string) (bool, error) {
 	k := table + "/" + chain
 	if rules, ok := n.n[k]; ok {
-		for _, rule := range rules {
-			if rule == strings.Join(args, " ") {
-				return true, nil
-			}
+		if slices.Contains(rules, strings.Join(args, " ")) {
+			return true, nil
 		}
 		return false, nil
 	} else {