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/iptables.go

@@ -21,8 +21,8 @@ import (
 
 func init() {
 	isNotExistError = func(err error) bool {
-		var e *iptables.Error
-		return errors.As(err, &e) && e.IsNotExist()
+		e, ok := errors.AsType[*iptables.Error](err)
+		return ok && e.IsNotExist()
 	}
 }