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)