refactor: move from io/ioutil to io and os package (#10341)

## Description

The `io/ioutil` package has been deprecated in Go 1.16 (See https://golang.org/doc/go1.16#ioutil). Since cosmos-sdk has upgraded to Go 1.17 (#9987), this PR replaces the existing `io/ioutil` functions with their new definitions in `io` and `os` packages.

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [x] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

testutil/ioutil.go

@@ -3,7 +3,6 @@ package testutil
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 	"os"
 	"strings"
 	"testing"
@@ -45,8 +44,8 @@ func ApplyMockIODiscardOutErr(c *cobra.Command) BufferReader {
 	mockIn := strings.NewReader("")
 
 	c.SetIn(mockIn)
-	c.SetOut(ioutil.Discard)
-	c.SetErr(ioutil.Discard)
+	c.SetOut(io.Discard)
+	c.SetErr(io.Discard)
 
 	return mockIn
 }
@@ -68,7 +67,7 @@ func WriteToNewTempFile(t testing.TB, s string) *os.File {
 func TempFile(t testing.TB) *os.File {
 	t.Helper()
 
-	fp, err := ioutil.TempFile(t.TempDir(), "")
+	fp, err := os.CreateTemp(t.TempDir(), "")
 	require.NoError(t, err)
 
 	return fp

testutil/ioutil_test.go

@@ -1,7 +1,8 @@
 package testutil_test
 
 import (
-	"io/ioutil"
+	"io"
+	"os"
 	"testing"
 
 	"github.com/spf13/cobra"
@@ -28,7 +29,7 @@ func TestWriteToNewTempFile(t *testing.T) {
 	tempfile := testutil.WriteToNewTempFile(t, "test string")
 	tempfile.Close()
 
-	bs, err := ioutil.ReadFile(tempfile.Name())
+	bs, err := os.ReadFile(tempfile.Name())
 	require.NoError(t, err)
 	require.Equal(t, "test string", string(bs))
 }
@@ -39,6 +40,6 @@ func TestApplyMockIODiscardOutErr(t *testing.T) {
 
 	testutil.ApplyMockIODiscardOutErr(cmd)
 	require.NotEqual(t, cmd.InOrStdin(), oldStdin)
-	require.Equal(t, cmd.OutOrStdout(), ioutil.Discard)
-	require.Equal(t, cmd.ErrOrStderr(), ioutil.Discard)
+	require.Equal(t, cmd.OutOrStdout(), io.Discard)
+	require.Equal(t, cmd.ErrOrStderr(), io.Discard)
 }

testutil/rest.go

@@ -1,7 +1,7 @@
 package testutil
 
 import (
-	"io/ioutil"
+	"io"
 	"net/http"
 )
 
@@ -25,7 +25,7 @@ func GetRequestWithHeaders(url string, headers map[string]string) ([]byte, error
 		return nil, err
 	}
 
-	body, err := ioutil.ReadAll(res.Body)
+	body, err := io.ReadAll(res.Body)
 	if err != nil {
 		return nil, err
 	}

testutil/rest/rest.go

@@ -5,7 +5,7 @@ package rest
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 )
 
@@ -18,7 +18,7 @@ func GetRequest(url string) ([]byte, error) {
 	}
 	defer res.Body.Close()
 
-	body, err := ioutil.ReadAll(res.Body)
+	body, err := io.ReadAll(res.Body)
 	if err != nil {
 		return nil, err
 	}
@@ -35,7 +35,7 @@ func PostRequest(url string, contentType string, data []byte) ([]byte, error) {
 	}
 	defer res.Body.Close()
 
-	bz, err := ioutil.ReadAll(res.Body)
+	bz, err := io.ReadAll(res.Body)
 	if err != nil {
 		return nil, fmt.Errorf("error reading response body: %w", err)
 	}