Replace deprecated ioutil with io and os (#20776)

* Replace ioutil with io and os

* Replace ioutil in utils/file.go

* Minor fix to tests and excluded files

Co-authored-by: Tim Scheuermann <tim.scheuermann@mattermost.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/response_transfer.go

@@ -6,7 +6,7 @@ package app
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"strconv"
 	"strings"
@@ -59,7 +59,7 @@ func (rt *PluginResponseWriter) GenerateResponse() *http.Response {
 	res.Status = fmt.Sprintf("%03d %s", res.StatusCode, http.StatusText(res.StatusCode))
 
 	if rt.Len() > 0 {
-		res.Body = ioutil.NopCloser(rt)
+		res.Body = io.NopCloser(rt)
 	} else {
 		res.Body = http.NoBody
 	}