all: replace non-trivial uses of package ioutil with os (#24886)

Co-authored-by: Martin Holst Swende <martin@swende.se>

rpc/server_test.go

@@ -20,7 +20,6 @@ import (
 	"bufio"
 	"bytes"
 	"io"
-	"io/ioutil"
 	"net"
 	"os"
 	"path/filepath"
@@ -53,7 +52,7 @@ func TestServerRegisterName(t *testing.T) {
 }
 
 func TestServer(t *testing.T) {
-	files, err := ioutil.ReadDir("testdata")
+	files, err := os.ReadDir("testdata")
 	if err != nil {
 		t.Fatal("where'd my testdata go?")
 	}