Warn if server version is less than installed version.

I am in China. Usually, I need to use mirror to update juliaup. Some mirrors fail to update for a long time resulting my juliaup downgrading when I want to upgrade.

I think it is a good idea to warn users if the latest version from server is less than installed version so that users know the present mirror server is not updating.

The default behavior should be not doing anything after warning since the command is `juliaup self update` rather than something like `juliaup self sync version`.

Start by tracing the implementation of `juliaup self update` and how it obtains the server and installed versions. Check the existing self-update tests, if present, and verify that a lower server version produces a warning while leaving the installed version unchanged.

Written by the indexing model from the issue text.