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/e2epostprocess/main.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"flag"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -48,7 +47,7 @@ func main() {
 	}
 	var found int
 	for _, file := range files {
-		data, err := ioutil.ReadFile(file)
+		data, err := os.ReadFile(file)
 		fatalOnError(err)
 		measurements := bytes.Split(data, []byte("\n"))
 		for _, measurement := range measurements {

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
 }
 

internal/cmd/miniooni/libminiooni.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"net/url"
 	"os"
@@ -258,7 +257,7 @@ func canOpen(filepath string) bool {
 
 func maybeWriteConsentFile(yes bool, filepath string) (err error) {
 	if yes {
-		err = ioutil.WriteFile(filepath, []byte("\n"), 0644)
+		err = os.WriteFile(filepath, []byte("\n"), 0644)
 	}
 	return
 }