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/

internal/cmd/jafar/main.go

@@ -6,7 +6,6 @@ import (
 	"errors"
 	"flag"
 	"fmt"
-	"io/ioutil"
 	"net"
 	"net/http"
 	"os"
@@ -185,11 +184,11 @@ func badProxyStartTLS() net.Listener {
 	proxy := badproxy.NewCensoringProxy()
 	listener, cert, err := proxy.StartTLS(*badProxyAddressTLS)
 	runtimex.PanicOnError(err, "proxy.StartTLS failed")
-	err = ioutil.WriteFile(*badProxyTLSOutputCA, pem.EncodeToMemory(&pem.Block{
+	err = os.WriteFile(*badProxyTLSOutputCA, pem.EncodeToMemory(&pem.Block{
 		Type:  "CERTIFICATE",
 		Bytes: cert.Raw,
 	}), 0644)
-	runtimex.PanicOnError(err, "ioutil.WriteFile failed")
+	runtimex.PanicOnError(err, "os.WriteFile failed")
 	return listener
 }