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/plugin_requests.go

@@ -6,7 +6,7 @@ package app
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"path"
 	"path/filepath"
@@ -157,11 +157,11 @@ func (ch *Channels) servePluginRequest(w http.ResponseWriter, r *http.Request, h
 			sentToken := ""
 
 			if r.Header.Get(model.HeaderCsrfToken) == "" {
-				bodyBytes, _ := ioutil.ReadAll(r.Body)
-				r.Body = ioutil.NopCloser(bytes.NewBuffer(bodyBytes))
+				bodyBytes, _ := io.ReadAll(r.Body)
+				r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
 				r.ParseForm()
 				sentToken = r.FormValue("csrf")
-				r.Body = ioutil.NopCloser(bytes.NewBuffer(bodyBytes))
+				r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
 			} else {
 				sentToken = r.Header.Get(model.HeaderCsrfToken)
 			}