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/image_test.go

@@ -4,7 +4,7 @@
 package api4
 
 import (
-	"io/ioutil"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -89,7 +89,7 @@ func TestGetImage(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		respBody, err := ioutil.ReadAll(resp.Body)
+		respBody, err := io.ReadAll(resp.Body)
 		require.NoError(t, err)
 		assert.Equal(t, "success", string(respBody))