Prompt for auto-update of versions in manifests

[davidanthoff]

How about this: when Juliaup uses a Project/Manifest for Julia selection, it also reads the [compat] section for julia from the Project.toml. It then checks whether there is a newer Julia version that is compatible via our normal semver rules. If that is the case, it prompts the user with something like:

This project is currently using Julia 1.12.1. Do you want to update the project to use Julia 1.12.5?
- Yes
- No
- No, and fix the project to Julia 1.12.1

If the user picks yes, Juliaup launches the correct Julia version, and triggers a project resolve, thus updating the manifest.
If the user picks no, we just launch the old Julia version
If the user picks No, and..., then we update the Project.toml [compat] section to read julia="=1.12.1", which should then prevent future update nags.

If the update would amount to a minor version jump, we could also offer options that fix the compat to minor version upgrades etc.

Originally posted by @davidanthoff in #1410 (comment)

Pulling this out into its own issue, not just a comment on a fairly unrelated PR.