Add weekly version bump CI job

[Liam-DeVoe]

This CI job should do two things:

• Update Cargo.toml with the latest dependency versions. Open a PR if anything changed.
• Check if any of our dependencies have released a new breaking version.
  • For post-1.0.0 libraries, that means a major version bump. For pre-1.0.0 libraries, that means a minor version bump.
  • Open a PR bumping that dependency if so. We expect this PR to fail CI, but it is a notification to us that we need to support the new library version.

The PR and commit author should be the hegel-releases app.

Related: #161, which I would love to have known about ahead of time.