Update length of cron tasks (#578)

- Bump amount of cron tasks that the dev image of Gitea has to 21.
- Fix TestAdminCronTasks.

Reviewed-on: https://gitea.com/gitea/go-sdk/pulls/578
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-committed-by: Gusted <williamzijl7@hotmail.com>

gitea/admin_test.go

@@ -45,7 +45,7 @@ func TestAdminCronTasks(t *testing.T) {
 
 	tasks, _, err := c.ListCronTasks(ListCronTaskOptions{})
 	assert.NoError(t, err)
-	assert.Len(t, tasks, 19)
+	assert.Len(t, tasks, 21)
 	_, err = c.RunCronTasks(tasks[0].Name)
 	assert.NoError(t, err)
 }