Use new methods in the io and os packages instead of ioutil ones

ioutil is deprecated in Go 1.16 and SFTPGo is an application, not
a library, we have no reason to keep compatibility with old Go
versions.

Go 1.16 fix some cifs related issues too.

httpdtest/httpdtest.go

@@ -7,7 +7,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"path"
@@ -817,7 +816,7 @@ func checkResponse(actual int, expected int) error {
 }
 
 func getResponseBody(resp *http.Response) ([]byte, error) {
-	return ioutil.ReadAll(resp.Body)
+	return io.ReadAll(resp.Body)
 }
 
 func checkFolder(expected *vfs.BaseVirtualFolder, actual *vfs.BaseVirtualFolder) error {