cleanup(all): stop using deprecated ioutil functions (#381)

Spotted while working on https://github.com/ooni/probe/issues/1417

See https://golang.org/pkg/io/ioutil/

cmd/ooniprobe/internal/ooni/ooni.go

@@ -265,7 +265,7 @@ func InitDefaultConfig(home string) (*config.Config, error) {
 	if err != nil {
 		if os.IsNotExist(err) {
 			log.Debugf("writing default config to %s", configPath)
-			if err = ioutil.WriteFile(configPath, defaultConfig, 0644); err != nil {
+			if err = os.WriteFile(configPath, defaultConfig, 0644); err != nil {
 				return nil, err
 			}
 			// If the user did the informed consent procedure in