Update petgraph requirement from ~0.5 to ~0.6

Updates the requirements on [petgraph](https://github.com/petgraph/petgraph) to permit the latest version.
- [Release notes](https://github.com/petgraph/petgraph/releases)
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](https://github.com/petgraph/petgraph/compare/0.5.0...0.6.0)

---
updated-dependencies:
- dependency-name: petgraph
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

db/Cargo.toml

@@ -16,7 +16,7 @@ lazy_static = "~1.4"
 log = "~0.4"
 ordered-float = "~2.5"
 time = "~0.2"
-petgraph = "~0.5"
+petgraph = "~0.6"
 serde = { version = "~1.0", optional = true }
 serde_json = { version = "~1.0", optional = true }
 serde_derive = { version = "~1.0", optional = true }