Rust crate dependency updates in `rust/` go unnoticed

[dak2]

Summary

The crate dependencies under rust/ (ruby-rbs, ruby-rbs-sys) are never updated automatically, and nothing notifies us when a new version is released.

Details

Current direct dependencies

Crate	Requirement	Locked version	Latest on crates.io	Used in
xxhash-rust	0.8	0.8.15	0.8.18	ruby-rbs
serde	1.0	1.0.229	1.0.229	ruby-rbs (build)
serde_yaml	0.9	0.9.34+deprecated	0.9.34+deprecated (unmaintained rustsec/advisory-db#2132)	ruby-rbs (build)
tempfile	3	3.27.0	3.27.0	ruby-rbs (dev)
bindgen	0.72.0	0.72.0	0.73.2	ruby-rbs-sys (build)
cc	1.2.29	1.2.29	1.4.7	ruby-rbs-sys (build)

None of the direct dependencies is a major version behind yet, but if one becomes so in the future, there is currently nothing that would make us notice.

Proposed fix

Add a cargo ecosystem to .github/dependabot.yml for /rust, so crate updates open PRs weekly like the other ecosystems.
Maybe, It seems there is room for debate as to whether “weekly” is the appropriate frequency.

Not covered

Replacing serde_yaml.
This is also a problem, so it would be better to replace it with a new one.

[dak2]

At first, I was thinking of doing it weekly, but I decided to switch to daily.
Since it's an automated merge, daily seems easier to handle.
#3157