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>

plugin/plugintest/example_hello_user_test.go

@@ -5,7 +5,7 @@ package plugintest_test
 
 import (
 	"fmt"
-	"io/ioutil"
+	io "io"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -52,7 +52,7 @@ func Example() {
 	r := httptest.NewRequest("GET", "/", nil)
 	r.Header.Add("Mattermost-User-Id", user.Id)
 	p.ServeHTTP(&plugin.Context{}, w, r)
-	body, err := ioutil.ReadAll(w.Result().Body)
+	body, err := io.ReadAll(w.Result().Body)
 	require.NoError(t, err)
 	assert.Equal(t, "Welcome back, billybob!", string(body))
 }