Remove ioutil (#181)

In 1.16, it's now deprecated and replaced by `io` and `os`.  Let's
upgrade!

Test plan:
- make check
- git grep ioutil

internal/integration/roundtrip.go

@@ -8,7 +8,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"testing"
 
@@ -29,13 +29,13 @@ func (t *lastResponseTransport) RoundTrip(req *http.Request) (*http.Response, er
 		return resp, err
 	}
 	defer resp.Body.Close()
-	body, err := ioutil.ReadAll(resp.Body)
+	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return resp, fmt.Errorf("roundtrip failed: unreadable body: %w", err)
 	}
 	t.lastResponseBody = body
 	// Restore the body for the next reader:
-	resp.Body = ioutil.NopCloser(bytes.NewBuffer(body))
+	resp.Body = io.NopCloser(bytes.NewBuffer(body))
 	return resp, err
 }
 

internal/integration/util.go

@@ -3,7 +3,6 @@ package integration
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -42,7 +41,7 @@ func RunGenerateTest(t *testing.T, relConfigFilename string) {
 	}
 
 	for filename, content := range generated {
-		expectedContent, err := ioutil.ReadFile(filename)
+		expectedContent, err := os.ReadFile(filename)
 		if err != nil {
 			t.Fatal(err)
 		}