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 {