Change the default cron schedule for mirroring

[minwoox]

Currently, the mirroring job of a project runs every minute if the schedule is not specified in the mirror config. When considering the infrequent changes to the configuration files, it seems that it is being called too frequently.

I think one of the reasons that we set it as every minute is that we didn't have any API that triggered the mirroring immediately. However, after we add that API, I think we can increase the time to 3 or 5 minutes.

Reference:

• ArgoCD uses 3 minutes as the default https://argo-cd.readthedocs.io/en/stable/user-guide/auto_sync/#:~:text=The%20automatic%20sync%20interval%20is,to%20180s%20(3%20minutes).