Refactor version handling in self_update to report upgradable version

[miried]

Summary

This change is an improvement on the dry_run functionality introduced in #9829. It now shows the version that would be upgraded to.

Note that despite its name, updater.is_update_needed() does not only check the version numbers, but it also checks if the executable can be actually upgraded. This is reflected in the wording of the new message. See https://docs.rs/axoupdater/0.9.0/axoupdater/struct.AxoUpdater.html#method.is_update_needed

Test Plan

Tested locally

info: Current version: v0.7.13
info: Checking for updates...
info: Latest/requested version: v0.7.13
info: Would not update uv. Already on the latest/requested version of uv.