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>

wgengine/router/osrouter/router_linux_test.go

@@ -1073,11 +1073,9 @@ func (o *fakeOS) run(args ...string) error {
 
 	switch args[2] {
 	case "add":
-		for _, el := range *ls {
-			if el == rest {
-				o.t.Errorf("can't add %q, already present", rest)
-				return errors.New("already exists")
-			}
+		if slices.Contains(*ls, rest) {
+			o.t.Errorf("can't add %q, already present", rest)
+			return errors.New("already exists")
 		}
 		*ls = append(*ls, rest)
 		sort.Strings(*ls)

wgengine/router/osrouter/runner.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"slices"
 	"strconv"
 	"strings"
 	"syscall"
@@ -42,8 +43,7 @@ func errCode(err error) int {
 	if err == nil {
 		return 0
 	}
-	var e *exec.ExitError
-	if ok := errors.As(err, &e); ok {
+	if e, ok := errors.AsType[*exec.ExitError](err); ok {
 		return e.ExitCode()
 	}
 	s := err.Error()
@@ -96,12 +96,7 @@ func newRunGroup(okCode []int, runner commandRunner) *runGroup {
 
 func (rg *runGroup) okCode(err error) bool {
 	got := errCode(err)
-	for _, want := range rg.OkCode {
-		if got == want {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(rg.OkCode, got)
 }
 
 func (rg *runGroup) Output(args ...string) []byte {

wgengine/router/router_test.go

@@ -19,8 +19,8 @@ func TestConfigEqual(t *testing.T) {
 	}
 	configType := reflect.TypeFor[Config]()
 	configFields := []string{}
-	for i := range configType.NumField() {
-		configFields = append(configFields, configType.Field(i).Name)
+	for field := range configType.Fields() {
+		configFields = append(configFields, field.Name)
 	}
 	if !reflect.DeepEqual(configFields, testedFields) {
 		t.Errorf("Config.Equal check might be out of sync\nfields: %q\nhandled: %q\n",