Add a CI workflow to run cargo-outdated checks

[gendx]

The cargo-outdated tool checks for outdated dependencies in a Cargo.toml. As a complement to cargo-audit which checks for vulnerable dependencies, we should leverage that to make sure dependencies are up-to-date.

[jmichelp]

Without the --depth argument, it's too verbose.

But this seems to be fine:

Name     Project  Compat  Latest  Kind         Platform
----     -------  ------  ------  ----         --------
elf2tab  0.4.0    ---     0.6.0   Development  ---

[jmichelp]

Blocked by kbknapp/cargo-outdated#241

Not only the produced JSON output is truncated (resulting in an empty JSON when it shouldn't be for example) but the produced JSON is also changing from a run to another.

[kaczmarczyck]

Now that we pin Cargo.lock files, we don't want outdated notifications. Instead, we will upgrade compiler versions and packages before a release (or earlier, if cargo audit notifies us of problems).

Will be fixed with #663 .