Switch bumping `cargo` security versions to using `cargo`'s `--precise` flag

We want to move toward leveraging native package manager functionality where possible rather than re-implementing it in Ruby. Beyond being more DRY, it generally lets us sidestep a lot of maintenance / edge-case bugs.

It looks like `cargo update` supports [a `--precise` flag that bumps a dependency to a specific version number](https://doc.rust-lang.org/cargo/commands/cargo-update.html).

We should explore switching to that for bumping security vulnerabilities.

Today we do the following for rust security updates:
1. query https://crates.io/ for available versions
2. filter for non-vulnerable versions
3. find the lowest remaining version
4. use Ruby `gsub()` to bump that in the manifest/lockfile

We should flip step 4 to using `cargo` directly via this `--precise` flag.

Related:
* https://github.com/dependabot/dependabot-core/issues/6426