cargo upgrade should default to --skip-compatible

For library crates, updating all dependency requirements in `Cargo.toml` to the latest semver-compatible versions is harmful to the ecosystem, as this makes it harder to resolve versions in case of for example yanking. I was already using `--skip-compatible` for my project, but just noticed that [another library crate](https://github.com/mattsse/chromiumoxide/pull/79) used `cargo upgrade` to force dependency upgrades this way, making it impossible to use the latest version of that crate after the latest futures version was [yanked](https://github.com/rust-lang/futures-rs/issues/2529).

Start at the `cargo upgrade` command and review how it chooses default options for library crates. Confirm the behavior around `--skip-compatible`, then add coverage showing that the default preserves compatible dependency requirements while the explicit option remains supported.

Written by the indexing model from the issue text.