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/imaging/utils_test.go

@@ -6,7 +6,6 @@ package imaging
 import (
 	"bytes"
 	"image/color"
-	"io/ioutil"
 	"os"
 	"testing"
 
@@ -77,7 +76,7 @@ func TestFillImageTransparency(t *testing.T) {
 			require.NotNil(t, inputImg)
 			require.Equal(t, "png", format)
 
-			expectedBytes, err := ioutil.ReadFile(imgDir + "/" + tc.outputName)
+			expectedBytes, err := os.ReadFile(imgDir + "/" + tc.outputName)
 			require.NoError(t, err)
 
 			FillImageTransparency(inputImg, tc.fillColor)