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>

api4/integration_action_test.go

@@ -5,7 +5,7 @@ package api4
 
 import (
 	"encoding/json"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -21,7 +21,7 @@ type testHandler struct {
 }
 
 func (th *testHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	bb, err := ioutil.ReadAll(r.Body)
+	bb, err := io.ReadAll(r.Body)
 	assert.NoError(th.t, err)
 	assert.NotEmpty(th.t, string(bb))
 	var poir model.PostActionIntegrationRequest